Mendelism: New Insights from Gregor Mendel’s Lectures in Brno

Abstract

After dissolving misinterpretation and prejudice of the newspaper report of Mendel’s lectures, we found the lecture content can be recovered...

Interpretation of Gregor Mendel’s work has previously been based on study of his published paper “Experiments in Plant Hybridization.” In contrast, the lectures that he gave preceding publication of this work have been largely neglected for more than 150 years. Here, we report on and interpret the content of Mendel’s previous two lectures, as they were reported in a local newspaper. We comprehensively reference both the text of his paper and the historical background of his experiments. Our analysis shows that while Mendel had inherited the traditional research program on interspecific hybridization in plants, he introduced the novel method of ratio analysis for representing the variation of unit-characters among offspring of hybrids. His aim was to characterize and explain the developmental features of the distributional pattern of unit-characters in two series of hybrid experiments, using self-crosses and backcrosses with parents. In doing so, he not only answered the question of what the unit-characters were and the nature of their hierarchical classification, but also successfully inferred the numerical principle of unit-character transmission from generation to generation. He also established the nature of the composition and behaviors of reproductive cells from one generation to the next. Here we highlight the evidence from Mendel’s lectures, clearly announcing that he had discovered the general law of cross-generation transmission of unit-characters through reproductive cells containing unit-factors. The recovered content of these previous lectures more accurately describes the work he performed with his garden peas than his published paper and shows how he first presented it in Brno. It is thus an invaluable resource for understanding the origin of the science of genetics.

GREGOR Mendel’s classic paper entitled “Experiments in Plant Hybridization” published in 1866 has long been the primary source for understanding Mendel’s work and exploring the origin of genetics (Bateson 1909; Stern and Sherwood 1966; Abbott and Fairbanks 2016). In contrast, Mendel’s prior lectures on his findings have been essentially ignored. It is well known that after finishing 8 years of hybridization experiments in Pisum, Mendel delivered two lectures to report his work at the meeting of the local scientific society in Brno on February 8 and March 8, 1865, then published his paper in the transactions of the society 1 year later (Mendel 1866). Mendel’s use of a draft outline of his lectures has been corroborated by his letter to Carl Nägeli sent on April 18, 1867. He wrote, “The paper which was submitted to you is the unchanged reprint of the draft of the lecture mentioned; thus the brevity of the exposition, as is essential for a public lecture” (Correns 1905; Mendel 1950). Regrettably, the draft was not preserved, leaving no record of Mendel’s own words regarding his lectures. Fortunately, an anonymous reporter of the Brno daily newspaper had briefly reported on the content of the two lectures as well as the reactions of the audiences, this record thus providing a valuable outline of what Mendel had expressed in the lecture room at that time.

As for earlier reports of the two lectures, there are two somewhat inadequate English versions available. English evolutionary embryologist Sir Gavin de Beer and American professor Robert Olby had each independently translated the German-language reports into English in the 1960s (de Beer 1966; Olby and Gautrey 1968). Remarkably, however, the two translators both discounted the value of the newspaper report. Sir Gavin considered the person who wrote the reports to be familiar with biological subjects but drew the incorrect conclusion that “the paper as published in 1866 contained exactly what Mendel said in 1865,” concluding that the newspaper accounts had little value (de Beer 1966). His words had a strong influence on the perception that “Mendel presented the results of his experiments at meetings … in Brno and handed over the manuscript for publication without alteration” (Orel and Hartl 1994). On the face of it, this is questionable: as an industrious scientist, Mendel was unlikely to spend a whole year waiting for the publication of his work without any attempt to modify the manuscript. In contrast, Olby regarded the writer of the reports to be a layman in botany, because he had noticed such unprofessional words as “male pollen” (“männlicher Blütenstaub” in German) in the report content (Olby and Gautrey 1968). When Olby encountered a confusing sentence, i.e., “characters common to both stem-parents were transmitted reciprocally, but differing characters gave rise to new characters,” he complained that the writer had taken inadequate notes and thus ascribed the statement to “the results being a garbled mixture of several distinct processes” (Olby and Gautrey 1968). Olby seems to have rather despised the writer, stating “he describes the numerical data as ‘beachtungswert’ (‘notable’ in English), which is more than either Hoffmann or Focke was prepared to say” (Olby and Gautrey 1968). Olby assessed Hoffmann and Focke as having more insight than the newspaper reporter, but he ultimately dismissed all 11 writers who referenced Mendel’s work before 1900, including the two botanists and the newspaper reporter, saying that “none really understood Mendel’s theory” (Olby and Gautrey 1968). Olby further criticized the reporter by saying that “so far as we know his reports led no one to Mendel’s ‘Versuche’” (Olby and Gautrey 1968). Indeed, as will be discussed in this article, the confusing sentence containing the term “new characters” is also found in Mendel’s paper, strongly suggesting that the writer had faithfully recorded what Mendel had said. Moreover, it is unlikely that Mendel would have permitted his lectures to be reported with no check of the content, because he had an outline of the lectures to help guide the newspaper reporter’s writing. Once the misunderstandings of the two translators are corrected, the newspaper report appears to be sufficiently valuable to merit further exploration, albeit it contains a few unprofessional words that should be noted.

Except for some disparities in the specific phrases and their meanings, the two English versions of the newspaper report are largely consistent with each other, and each can thus provide adequate information to reconstruct the putative themes of the two lectures, as shown below. Since Olby’s version is more perceptive than de Beer’s, however, here we will refer to the former more often when trying to recover the content of Mendel’s lectures in Brno. By comprehensively referencing the content in Mendel’s publication, together with the cumulative findings of historian research into the background of his experiments, especially Vítězslav Orel’s findings (Paleček 2016), we offer a reinterpretation of the two reports from as objective a perspective as possible. In this study, Mendel’s published article is referred to as “Versuche über Pflanzen-Hybriden” (Mendel 1866), abbreviated as “the Versuche,” and the English versions of Mendel’s paper that are referred to are those of William Bateson’s 1909 publication (Bateson 1909) and Scott Abbott and Daniel J. Fairbanks’ 2016 publication (Abbott and Fairbanks 2016).

Report of the First Lecture (Brno Feb. 8, 1865): The Analysis of Unit-Characters and the Tendency Toward Reversion of Hybrids in Successive Generations

1) After the reading of the communications received, Herr Professor G. Mendel delivered a long lecture, of special interest to botanists, on plant hybrids raised by artificial fertilization of related species.

This is a general introduction of Mendel’s study concerning the hybrids in Pisum. In present-day usage, the term “hybrid” has two meanings in biology. For evolutionary biologists, it is restricted to organisms formed by cross-fertilization between individuals of different species. For breeders or geneticists, however, “hybrids” are more broadly regarded as the offspring between individuals from populations that are distinguishable in one or more heritable characters (Rieseberg 1997). Here, “artificial fertilization of related species” definitely referred to interspecies or intervarieties hybridization, so “hybrid” was a kind of evolutionary concept, consistent with the term’s use in the research of J. G. Kölreuter and C. F. von Gärtner, which was frequently cited in the Versuche (Mendel 1866). S. Müller-Wille regarded Mendel as a member of the club of traditional hybridism and ascribed his shift of the subject of experiments from that of hybrid forms to that of individual characters to a dynamical development of the experimental program of the hybridist tradition (Müller-Wille 2007). The question then becomes: how did Mendel tell the story of hybrids in this new sense, a meaning which has since been adopted by all geneticists?

2) The lecturer emphasized that the fertility of the plant hybrids or crossbreds was proven but did not remain constant, and that these hybrids always tended to revert to the stem species, this reversion being sped up by repeated artificial fertilizations with the pollen of the stem plants.

Mendel admitted that the work of the German scientist Kölreuter on plant hybridization had played an important role in the development of his own research (Mendel 1866). Callender (1988) summarized Kölreuter’s main findings as follows: “hybrids between species were invariably sterile whilst hybrids between varieties, if they manifested any degree of fertility, would always be extinguished in the course of time as a result of the twin processes of Reversion and Transformation.” The phenomenon of “Reversion” referred to the tendency of offspring of a self-fertilized hybrid to be more inclined in appearance to one or the other of the parental forms than to the hybrid itself, and “Transformation” referred to the potential for species A to be transformed into species B by repeated backcrosses of offspring (Callender 1988). It is clear that Mendel had the same research interest as his predecessor Kölreuter in hybrid fertility and in the tendency of hybrids to revert or transform in subsequent crosses. Mendel used the same experimental framework as his predecessor, performing the self-crosses of hybrids in multiple generations that resulted in a tendency toward (trait) “Reversion” and also conducting backcrosses with parents that brought about (trait) “Transformation” in successive generations. The two series of experiments can be concluded to have been simultaneously initiated and conducted in parallel, because the use of the term “being sped up” in the newspaper report clearly reveals that Mendel had compared the tendencies of “the twin processes” against each other in practice. Particularly, Mendel may have succeeded in setting up a novel method to accurately measure the speed of reversion. On the other hand, the main conclusion of his study was presented in advance at the beginning of the lectures, in a way intended to excite the audiences’ interest. What was this novel method? How did Mendel define the reversion speed, and why did he indicate that the reversion rate through backcrosses of hybrids was faster than that in self-cross experiments? Mendel did so by introducing his novel method of analyzing unit-characters to define and measure the reversion speed.

3) The lecturer drew attention to his experiments carried out over several years with success, which he had made especially with several kinds of pea (Pisum sativum, P. saccharatum and P. quadrature) and exhibited examples from the generations in question, in which characters common to both stem-parents were transmitted reciprocally, but differing characters gave rise to new characters.

This statement has considerable information content, but it also contains a puzzling expression as well. Here “Pisum sativum, P. saccharatum and P. quadrature” denote his research materials. The species of P. umbellatum, which has a terminal position of inflorescence, was also referred to in his paper (Mendel 1866). We can surely conclude that “his experiments carried out over several years with success” included “the twin experiments” that referred to both the self-cross and the backcrosses. This leaves the confusing claim that “differing characters gave rise to new characters,” as noted by Olby earlier (Olby and Gautrey 1968).

To avoid misunderstanding, the puzzling phrase “new characters” must be read in the context of the corresponding content in “Division and Arrangement of The Experiments” in the Versuche: “If two plants which differ constantly in one or several characters be crossed, numerous experiments have demonstrated that the common characters are transmitted unchanged to the hybrids and their progeny; but each pair of differentiating characters, on the other hand, unite in the hybrid to form a new character, which in the progeny of the hybrid is usually variable” (Mendel 1866). Apparently, the division of experiments and their descriptions in the paper are in agreement with the division of the experiments described in the lecture. Since the sentence belongs to the introductory paragraph of the section in the Versuche, it is reasonable to analyze the corresponding content of the lecture according to the context of the publication.

The words from this section demonstrate that Mendel’s research target was the paired characters carried by pea plants being crossed, later termed “unit-characters” by de Beer (1964), and divided into the common characters and differentiating characters in the first level (Figure 1). The differentiating characters, also known as opposing characters, differing characters, or contrasting characters, were such a significant term for Mendel and his audiences that it required detailed description and further classification into “qualitative characters” and “quantitative characters” in terms of its difference types in the second level (Figure 1). Mendel regarded quantitative traits as “the difference of a “more or less” nature and often difficult to define,” as exemplified in Pisum by the length of stems and flower stalks, as well as the size of leaves, flowers, seeds, and pods, etc.; and regarded qualitative traits as “characters which stand out clearly and definitely,” such as the color of stem, flowers, pods, seed coats and albumen (endosperm), the form of leaves, pods and seeds, and the position of flowers of garden pea, and so on (Mendel 1866). Further, Mendel was able to split the qualitative characters into the completely dominant ones and the incompletely dominant ones in the third level (Figure 1). In “The Forms of The Hybrids,” the former were described as that of “hybrid-character resembling that of one of the parental forms so closely that the other either escapes observation completely or cannot be detected with certainty,” while the latter were described as that of “the intermediate one” (Mendel 1866). Remarkably, this understanding of the assortment of unit-characters was achieved by Mendel quite clearly and quite early. Mendel’s hierarchical divisions of the paired characters, as deduced from the Versuche, can be listed as shown in Figure 1.

It should be noted here that while Mendel picked up the seven completely dominant characters to explore the nature of hybrids, he also took those more complex quantitative characters into account and studied them. Although the lengths of stem in garden pea apparently matched his discriminated criteria as “more or less” quantitative characters, the quality of hybrid vigor appeared in F₁ individuals for these traits. Nevertheless, the differences in the trait in the parents were so marked that Mendel treated it as one of the completely dominant characters, and it was later identified by D. R. Lester as a major effect gene le that encodes a protein capable of converting an inactive precursor, GA₂₀, to the bioactive GA₁ (Lester et al. 1997). Another typical continuously variable trait, flowering time in Pisum, was also investigated by Mendel in his later hybridization experiments. Despite the fact that the crops were heavily damaged by pea pests, as reported in his letter to Nägeli (Correns 1905; Mendel 1950), Mendel concluded with respect to the trait’s transmission: “the constitution of the hybrids with respect to this character probably follows the rule ascertained in the case of the other characters” (Mendel 1866). The words were intentionally placed in the section concerning “the several differentiating characters,” leading us to speculate that Mendel might have perceived what we today might term its “polygenic essence.” Additionally, on the basis of his observation that the degree of soil fertility affected the length of stem, along with the fact that the temperature and the seeds’ depth in the earth had an effect on germination time, then later on flowering time, Mendel was clearly aware of the role of the environment in affecting the variation of certain characters. Thus, Mendel not only concluded that quantitative characters shared the rules he had discovered for the qualitative characters he had studied in Pisum, but also discovered the role of environmental factors in modulating the traits.

In light of the hierarchical divisions described above, we can see that the puzzling sentence of the newspaper report corresponding to the statement in the Versuche does not uniquely refer to the self-cross of plants bearing any one kind of the differing characters. The incompletely dominant or quantitative characters yield the intermediate type in the F₁ generation, which can be regarded as “a new character.” In contrast, a completely dominant trait resembles a parental character in the F₁ hybrid, rather than “a new character” altogether. Thus, “a new character” does not refer to any concrete or observable characters derived from those of parents or produced by interactions between characters of both parents. Nevertheless, no matter what kind of differing characters of parents the F₁ hybrid is descended from, their offspring in the subsequent generations show the phenomenon of segregation, completely agreeing with the statement regarding “a new character, which in the progeny of the hybrid is usually variable.” Thus, here “a new character” means nothing more than “the variability of the offspring of hybrids,” a new property acquired from the reshuffling of unit-characters through hybridization, which the newspaper report recorded as “not remain(ing) constant” and substituted the terminology with “new characters,” which is different only in its plural form (de Beer 1966; Olby and Gautrey 1968). As shown in the Versuche, the nature of variability is such a core issue in Mendel’s study that “the object of the experiment was to observe these variations in the case of each pair of differentiating characters, and to deduce the law according to which they appear in successive generations” (Mendel 1866). This is perfectly consistent with its research aim and the subsequent execution of the experiments as presented in the lectures, namely to characterize and compare the distribution pattern of unit-characters across both types of hybridization experiments.

4) The differentiating characters of the pea hybrids were seen in the form and color of the ripe seed and seed coat, in the color of the flowers, in the form of the ripe pods and their color when unripe, in the position of the flowers and in the difference in length of the stems. The numerical data with regard to the occurrence of differentiating characters in the hybrid and their relation to the stem species was worthy of consideration.

Mendel selected completely dominant unit-characters to study not only because they could readily be visually discriminated (from the recessive forms) but also because they were easy to symbolize, hence denote. Nine unit-characters were originally chosen for his research and described in the lectures, but the form of seed coats and the color of the flowers did not appear in the Versuche. Why? Mendel noticed that white flowers were consistently associated with white seed coats, and purple flowers with gray seed coats, hence flower color and seed coat color are not independent characters (Mendel 1866). In addition, the form of seeds is determined by maternal tissue while flower color is determined by the embryo’s genetic constitution. Eventually, both of these characters, the color of flowers and the form of seed coat, were omitted from the paper.

Mendel claimed that his experiments in Pisum had been “carried out to such an extent and in such a way as to make it possible to determine the number of different forms under which the offspring of the hybrids appear, or to arrange these forms with certainty according to their separate generations, or definitely to ascertain their statistical relations” (Mendel 1866). Fulfilling these three tasks, all involving both self-crosses and backcrosses, demanded a large amount of experimental labor, and in the end, he needed 8 years to do the work. During this period, the experiments of self-pollination of hybrids had been carried through from four to six generations in Pisum for each unit-character, as is clearly presented in his published paper (Mendel 1866). Despite some vagueness in the number of generations, a large set of “transformation” experiments between two species in Pisum was recorded in Concluding Remarks of the Versuche. These embraced two types of backcross experiments, “the first experiment B/A” and “the second experiment A/B” (Mendel 1866), which can be calculated to have taken Mendel 4 years, at a minimum. The twin experiments were probably conducted in parallel for the sake of comparisons between the two reversion speeds (Figure 2).

With the aid of the established analytical method, the regular behavior of hybrid offspring could be detected from “the numerical data with regard to the occurrence of differentiating characters in the hybrid and their relation to the stem species.” Here, we adopt the same usage as Mendel’s in his paper, such that the capital A and the lowercase a were assigned to represent the dominant and the recessive of a pair of unit-characters. The different forms that appeared in offspring of hybrids, including two constant forms and one hybrid form, were also symbolized as A, a, and Aa, respectively. In this way, Mendel developed a simple notation that allowed him to quickly enumerate the results of his crosses in both kinds of experiments. The reversion speed was defined as the increase of the proportion of individuals carrying one of the parental characters (either dominant A or recessive a) in the offspring population of hybrids, in a given generation number. Thus, the reversion speed toward either a or A was determined to be 1 − 1/2ⁿ in backcross BC_I and BC_II, respectively, while the two speeds were uniformly (1 − 1/2ⁿ)/2 in the selfing series, where n represents the generation number (Figure 2). Comparatively, the former is certainly larger than the latter. Mendel explicitly stated, “this reversion being sped up by repeated artificial fertilizations with the pollen of the stem plants.”

In conclusion, Mendel inherited the traditional research program for investigating interspecific hybridization in plants, but went beyond it in characterizing the reversion tendency of unit-characters in the form of a mathematical expression. His achievement in doing so is attributable to his creative insight and to the alteration of his research target from the paired species examined by his predecessor Kölreuter to the paired unit-characters in his study. Indeed, Callender had already characterized the reversion tendency of hybrids detected by Kölreuter and Gäertner as “a fundamental law of nature” concerning the hybrid species, but regarded Mendel’s statement of inclinations toward reversion as an extension of “the Law of Simple Combination of Characters” (Callender 1988). Hence, the first lecture was very successful because his research target was both observable and novel, and his conclusion was confirmed as clear and acceptable. The reporter recorded “That the theme of the lecture was well chosen and the exposition of it entirely satisfactory was shown by the lively participation of the audience” (Olby and Gautrey 1968). Callender, however, pointed out that Köelreuter had earlier maintained a belief that parental pollens were more effective than hybrid pollen when both “reversion” and “transformation” existed jointly (Callender 1988); thus, the conclusion drawn by Mendel in this lecture primarily served to mathematically define the existing perspective of his predecessor, and its novelty can perhaps be therefore somewhat discounted.

Report of the Second Lecture (Brno Mar. 8, 1865): The Cross-Generation Principle Regarding Reproductive Cell Formation, Fertilization, and Seed Production

1) Taking up the thread of last month’s lecture he spoke about cell formation, fertilization and seed production in general, and in the case of hybrids in particular, alluding to his experiments undertaken with as much care as success, which he declared he would continue next summer.

The thread of Mendel’s first lecture was taken up to start his second lecture. Olby considered Mendel’s brief review was only “a demonstration of representatives from F₁ and F₂ generations” (Olby and Gautrey 1968). We consider besides these, however, that representations from BC₁ and BC₂ in both types of backcrosses, BC_I and BC_II, were also surely repeated. This consideration relies on the fact that the types and composition of pollen and egg cells of hybrids could only be inferred from the data of BC_I and BC_II, respectively (Figure 2), and Mendel was very familiar with this inferential method as shown in the second section of the Versuche (Mendel 1866). Indeed, de Beer also noticed that Mendel’s second lecture “began with the subject of germ-cells of the hybrids, on page 24 of his paper”; that is, exactly the beginning of the content concerned with the interpretation of the reproductive cells of hybrids (Mendel 1866; de Beer 1966). Although test crosses nowadays are usually described as Mendel’s special design to validate the alleged “hypothesis of segregation” in the Versuche, whether it emerged here or not is relatively insignificant because it is essentially a typical case of using a backcross.

Mendel altered his perspective in this lecture to examine the behavior of reproductive cells in the process of sexual reproduction. This alteration was full of challenges, because the bodies of germ-cells as well as their actions in fertilization were unobservable under the limited conditions available at that time. The alteration was so straightforward, however, that it simply required the symbols A and a being viewed as the potential elements carried by various germ-cells that determine the unit-characters of the plant, later termed “unit-factors” by de Beer (1964). Actually, the terms “factors,” “elements,” and “potentials” were frequently found in the Versuche (Mendel 1866); even “anlage,” very similar to the contemporary “gene,” appeared in his letter to Nägeli on September 27, 1870 (Correns 1905; Mendel 1950). Accordingly, the principle of “sex cell formation, fertilization, and seed production in the case of hybrids” must be concerned with the unit-factors in Mendel’s mind, and mathematically presented in the form: A/A + A/a + a/A + a/a = A + 2Aa + a, the same equation used in the Versuche to represent his laws of heredity (Hartl and Orel 1992). On the basis of the composition of A + a representing hybrid pollen cells and hybrid egg cells, together with the random manner of fertilization (Figure 2), the perfect square formula (A + a)² = A/A + A/a + a/A + a/a = A + 2Aa + a perfectly depicts the behaviors of reproductive cells during the process of reproduction. It is necessary to point out that Mendel’s employment of the same notations (A and a) to represent either unit-characters or unit-factors in his paper implied that he had not sharply distinguished between genotype and phenotype in his own mind, which is the only recognized conceptual fault in his work (Hartl and Orel 1992). Mendel had repeated this statement with minor variations no less than six times in the Versuche: “the pea hybrids form egg and pollen cells which, in their constitution, represent in equal numbers all constant forms which result from the combination of the characters united in fertilization” (Mendel 1866; Hartl and Orel 1992). Ultimately, it can be concluded that the discovery of the law of inheritance was just a process of mathematical modeling, albeit with a few variables lacking a clear-cut distinction, e.g., between “unit-characters” and “unit-factors,” that is, “phenotype” and “allele” in modern genetic parlance (Hartl and Orel 1992).

It is notable here that Mendel unambiguously declared that he had discovered the general principle governing the behaviors of reproductive cells in plant reproduction, a universally applicable law controlling unit-characters’ transmission from one generation to the next generation, which is now regarded as the fundamental law of inheritance. In striking contrast with his lecture, Mendel wrote in the Versuche that he just wanted to formulate a generally applicable law governing the formation and development of hybrids (Mendel 1866). Accordingly, the second lecture is the sole place where Mendel announced his discovery of the general law of inheritance. In this sense, the day of March 8 in 1865 can be considered to have marked the birth of genetics. Since then, also, the term “hybrid” has been given a new meaning in genetics, which was precisely derived from but differed from the traditional concept of the interspecific hybrid in evolutionary biology.

2) At the end he said that in the last few years he had also undertaken artificial fertilizations with many other related plants, which he named, in order to raise hybrids, and he felt encouraged by the favorable results achieved not only to experiment further with such hybridizations, but also to offer detailed reports.

Interestingly, de Beer found that Mendel had indicated an intention to continue his experiments on peas the next summer, but this statement was inconsistent with his publication, “the experiment...is now, after continuance during eight years, practically concluded” (de Beer 1966). In light of Mendel’s letter to Nägeli on April 18, 1867, the experiments using peas as materials were conducted from 1856 to 1863 (Correns 1905; Mendel 1950), a span of 8 years. In the same letter to Nägeli, Mendel said that for the sake of avoiding the dual hazards “for experimenter and for the case he represented,” he had undertaken a number of hybridization experiments with other plants in 1863 and 1864 to verify the results obtained with Pisum, but he complained that it was very difficult to find plants suitable for an extended series of such experiments (Correns 1905; Mendel 1950). In the Versuche, Mendel reported such minor experiments that had been carried out just before the publication of the paper in 1866. An experiment with Phaseolus vulgaris and Ph. nanus gave results in perfect agreement with those discovered in Pisum, while another experiment with Ph. nanus L. and Ph. multiflorus W. had only partially corresponding results (Mendel 1866). Here, Mendel might present he was working on such cases up until his lecture in 1865.

3) Herr Professor von Niessl added to this lecture which was very well received that with the aid of the microscope he had observed hybridizations in fungi, mosses and algae, and that further observations of this kind not only supported existing hypotheses but will also give further interesting clarifications.

It was said that Mendel’s second lecture was less well received by his audience, but “most interesting is the fact that when he had finished his second lecture it was discussed, contrary to what has previously been said about it” (de Beer 1966). Professor von Niessl orally supported Mendel’s claims regarding the behavior of reproductive cells in hybridization, by means of sharing his microscopical observations of fertilization in fungi, mosses, and algae. Indeed, experimental tests in this field were rather sparse at that time, and microscopic observations of meiosis of ongoing germ cell formation and double fertilization in angiosperms were not made until a few decades after Mendel’s discovery. Conceivably, the discussion was about the random pattern of one pollen uniting with one egg in fertilization, just as the equation implied. In the Concluding Remarks of the Versuche, Mendel wrote, “in the opinion of renowned physiologists, for the purpose of propagation one pollen cell and one egg cell unite in Phanerogams into a single cell, which is capable by assimilation and formation of new cells to become an independent organism” (Mendel 1866).

Owing to a lack of relevant evidence from cytological observation, the ultimate presentation of the second lecture only consisted of a hypothesis, rather than a confirmed conclusion. Mendel had no direct cytological observations to substantiate his claims, only the terms “factor,” “element,” and so on. The reporter faithfully evaluated the second lecture as a “vey well received speech” (“mit vielfacher Anerkennung belohnten Vortrage” in German) but indicated that further investigation was needed to clarify many uncertain points raised by the talk (Olby and Gautrey 1968). Mendel may have felt helpless and a bit disappointed when he faced somewhat negative reactions from the audience. Perhaps the audience’s changing attitude in the lecture room stimulated Mendel to adjust the presentation of his research program, as demonstrated by his published paper indicating that the principle of inheritance was primarily linked to the analysis of the evolutionary role of hybrids (Olby 1979).

Mendel’s division of his findings into two parts in his oral presentation was not due to instructions from the committee, but instead, was his own choice. The two lectures clearly focused on distinct aspects of his work and drew relevant conclusions with separate themes. The first lecture focused on the observation and counting of the offspring of hybrids carrying various unit-characters in Pisum and obtained the results at a macroscopic level, revealing the reversion tendency occurring in the proportional distribution of the dominant and/or the recessive trait in successive generations. Moreover, this research eventually resulted in the confirmation of a previously existing scientific opinion. In contrast, the second lecture focused on the unobserved germ-cells carrying unit-factors with the potential to determine unit-characters, along with their composition, proportion, and manner of union in fertilization. This lecture presented the general law of inheritance in a mathematical equation that described the process of plant reproduction and presented a groundbreaking discovery, despite its hypothetical nature. Both studies, however, were based on the same experimental data, concentrated on the same research target for understanding unit-characters (though at different levels), applied the same method for ratio analysis of the variation of unit-characters or unit-factors, and aimed to achieve the same research objective of understanding the transmission principle for unit-characters. The first was related to its developmental history, while the second was related to the inner determinants in the process of reproduction. The latter was a rational expansion of the former and served as a satisfactory explanation of the experimental results. Thus, the two parts were integrated into a hierarchical research program that had made rapid progress from macro to micro scales and from an exterior to an interior perspective. Comparatively speaking, although the second lecture only claimed an unconfirmed hypothesis in the form of an equation, it was not the end but the climax of the lecture series. The equation was the most creative element in the whole research program and was subsequently regarded as the law of inheritance (Hartl and Orel 1992).

“Mendel’s printed paper can easily be recognized in the report published in Neuigkeiten” (de Beer 1966). In retrospect, we can see that Mendel had inherited the traditional research on interspecific hybrids in plants but had created a novel method for the analysis of unit-characters. When he aimed to characterize and explain the developmental feature of pairs of unit-characters in two series of self-cross and backcross hybrids in successive generations, he not only succeeded in inferring the transmission principle of unit-characters from generation to generation, but also the law regarding the composition and behavior of reproductive cells from one generation to the next. Here, we have highlighted, through his lectures, Mendel’s announcement of his discovery of the general law of unit-character transmission across generations through reproductive cells containing unit-factors. This discovery justifies his honored position as the father of genetics.

Subsequently, when Mendel’s findings were published in 1866, some rearrangement and revision of the experiments were necessary to integrate the two lectures into a piece of cohesive writing. Indeed, alterations in his publication were noticed by his advocates, including Fisher (1936). To some extent, such alterations made by Mendel himself may be responsible for some long-term controversies regarding the interpretation of his discovery (Orel and Hartl 1994), even leading to the statement “whether he truly made the first move in revealing the underlying mechanism of heredity is still a matter of debate” (Orel and Wood 2000). This reflects the fact that the traditional analysis of Mendel’s work, hence the origin of genetics, was based on Mendel’s published article, while little attention was paid to his lectures (Bateson 1909; Stern and Sherwood 1966; Abbott and Fairbanks 2016). Once de Beer’s misinterpretation and Olby’s prejudiced consideration of the newspaper report of Mendel’s lectures are discounted, the content of the first lecture can be seen to more closely reveal the methodological approach underlying Mendel’s experimental work with the garden pea in Brno, while the content of the second lecture can be credited as a clear declaration of his first discovering the law of inheritance. Thus, it has been quite constructive to compare Mendel’s lectures and his later publication, the former revealing more clearly than the latter how Mendel understood the general significance of his work.

Acknowledgments

We thank Adam Wilkins and two anonymous reviewers for their contributions to this paper. We are grateful to Jiří Sekerák (Mendelianum Centre of Moravian Museum, Czech Rep.) and Eva Matalová (Institute of Animal Physiology and Genetics, Czech Rep.) for providing a copy of the articles on Mendel’s lectures in Neuigkeiten. We also thank Yongbiao Xue (Institute of Genetics and Developmental Biology, Chinese Academy of Science) for helpful discussions, and Agboyibor Clement (Ghanaian, Northwest Normal University, China) and Eunice Essel (Ghanaian, Gansu Agricultural University, China) for help with our English. This work was financially supported by the National Natural Science Foundation of China (31060033). The authors declare no conflicts of interest.

Footnotes

Literature Cited