Diverse Variation of Reproductive Barriers in Three Intraspecific Rice Crosses

Corresponding author: Yoshiaki Harushima, National Institute of Genetics, 1111 Yata, Mishima, Shizuoka 411-8540, Japan., yharushi{at}lab.nig.ac.jp (E-mail)

Communicating editor: C.-I WU

	ABSTRACT

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

Reproductive barriers are thought to play an important role in the processes of speciation and differentiation. Asian rice cultivars, Oryza sativa, can be classified into two main types, Japonica and Indica, on the basis of several characteristics. The fertility of Japonica-Indica hybrids differs from one cross to another. Many genes involved in reproductive barriers (hybrid sterility, hybrid weakness, and gametophytic competition genes) have been reported in different Japonica-Indica crosses. To clarify the state of Japonica-Indica differentiation, all reproductive barriers causing deviation from Mendelian segregation ratios in F₂ populations were mapped and compared among three different Japonica-Indica crosses: Nipponbare/Kasalath (NK), Fl1084/Dao Ren Qiao (FD), and Fl1007/Kinandang puti (FK). Mapping of reproductive barriers was performed by regression analysis of allele frequencies of DNA markers covering the entire genome. Allele frequencies were explained by 33 reproductive barriers (15 gametophytic and 18 zygotic) in NK, 32 barriers (15 gametophytic and 17 zygotic) in FD, and 37 barriers (19 gametophytic and 18 zygotic) in FK. The number of reproductive barriers in the three crosses was similar; however, most of the barriers were mapped at different loci. Therefore, these reproductive barriers formed after Japonica-Indica differentiation. Considering the high genetic similarity within Japonica and Indica cultivars, the differences in the reproductive barriers of each cross were unexpectedly numerous. The reproductive barriers of Japonica-Indica hybrids likely evolved more rapidly than other genetic elements. One possible force responsible for such rapid evolution of the barriers may have been the domestication of rice.

THE dynamics of speciation is one of the central topics of evolution, and study of the reproductive barriers that prevent potential or actual gene flow between differentiating lineages is crucial to the study of speciation. Several reproductive barrier genes have been cloned, and the molecular mechanism of reproductive isolation and the evolution of the genes involved have been studied extensively in abalone (VACQUIER et al. 1990 ; SWANSON and VACQUIER 1997 ), sea urchin (GAO et al. 1986 ), Chlamydomonas (FERRIS et al. 1997 ), Drosophila (TING et al. 1998 ; MERRILL et al. 1999 ), and mouse (HERRMANN et al. 1999 ). However, an advanced understanding of individual isolation mechanisms does not devalue the worth of studies on the state of multiple reproductive barriers between differentiating lineages, because reproductive isolation is a result of multiple genes acting at various stages throughout the life history of an organism.

A new method for genome-wide surveys of reproductive barriers causing deviation from Mendelian segregation ratios in F₂ progeny was reported previously (HARUSHIMA et al. 2001 ). As an example of the method, the reproductive barriers of an intraspecific hybrid between a Japonica rice cultivar, Nipponbare, and an Indica rice cultivar, Kasalath, were analyzed (HARUSHIMA et al. 2001 ). However, results obtained from a single cross do not represent the full range of differentiating Asian rice cultivars, Oryza sativa L., since varietal differentiation in O. sativa is highly complex. O. sativa can be classified into two main groups, Japonica and Indica, which differ in many respects (OKA 1988 ). For example, there is a high frequency of restriction fragment length polymorphism (RFLP) between Japonica and Indica cultivars (KURATA et al. 1994 ), while RFLP frequencies within Japonica or Indica cultivars are extremely low (ANTONIO et al. 1996 ). The status of the reproductive barriers between Japonica and Indica is currently unclear. Indica and Japonica types were originally defined as intersterile groups; however, the fertility of their hybrid offspring varies widely from one cross to another. Thus, the Asian varieties cannot be classified into two distinct types on the basis of F₁ sterility (OKA 1988 ). F₁ sterility is the result of an abortion of the gametophytes or zygotes and causes allele frequency distortions in the F₂ population. To clarify the status of Japonica-Indica differentiation, reproductive barriers causing deviation from Mendelian segregation ratios were mapped by a newly developed regression method and compared among three different Japonica-Indica crosses. This is the first study that compares multiple reproductive barriers dispersed throughout the genome in differentiating populations.

	MATERIALS AND METHODS

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

Plant material and map construction:
Nipponbare is a Japonica cultivar; Kasalath, Dao Ren Qiao, and Kinandang puti are Indica cultivars from India, China, and the Philippines, respectively. Fl1087 and Fl1007 are Japonica marker lines developed at Kyushu University, Fukuoka, Japan, and correspond to FL 102 and FL 7, respectively. Three F₂ populations, from crosses of Nipponbare and Kasalath (NK), Fl1087 and Dao Ren Qiao (FD), and Fl1007 and Kinandang puti (FK), were obtained by self-pollination of F₁ plants that were produced using Indica varieties as pollen parents. The seed fertility of the F₁ plants was normal (96%) in NK, but was 50% in FD and FK. Genetic linkage maps for NK, FD, and FK were constructed using 186, 94, and 93 F₂ plants, respectively. NK genetic map construction was described by KURATA et al. 1994 and HARUSHIMA et al. 1998 , and the maps of FD and FK were described by ANTONIO et al. 1996 . The positions of markers in the NK map were expressed as the accumulated marker distances from the short arm end of each chromosome. The marker positions in the other two maps were calculated from the reference marker held in common with the NK map and nearest to the short arm end.

Regression analysis:
Regression analyses for FD and FK were performed using each linkage map. For regression analysis, allele frequencies of cosegregating markers that were mapped at the same location were eliminated. For the FD map, the allele frequencies of 236 DNA markers, including 206 markers common to the NK map, were used. The average number of plants used for calculating allele frequencies was 93.87, and the minimum number of plants used was 88. In the FK map, 222 markers were used, including 190 markers common to the NK map. The average number of plants used to calculate allele frequency was 92.73, and the minimum number of plants used was 87 for the FK map. Mathematica packages were used for multiresponse nonlinear regression analyses to estimate the map position, intensity, and type (gametophytic or zygotic) of the reproductive barriers causing deviation from Mendelian segregation ratios on each chromosome (HARUSHIMA et al. 2001 ). Estimations were made by selecting the best-fit mathematical model to explain the observed allele frequencies of all the markers on a chromosome in an F₂ population. Occasionally, when the peak of an allele frequency appeared near the end of a chromosome, the regression analysis was executed using fixed parameter values for a given barrier to avoid diverging from the limits of chromosome length. For comparing barrier locations among different crosses, the NK map positions of the reproductive barriers detected in FD and FK were estimated proportionally using the barrier flanking markers common to the NK map.

	RESULTS

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

Allele frequencies and regression analysis:
Allele frequencies of DNA markers in the FD, NK, and FK maps were plotted along the respective genetic linkage maps of each chromosome (Fig 1). Deviations from the expected Mendelian segregation ratios (25% each homozygote and 50% heterozygote) were observed for all chromosomes except chromosome 7 of the FD map. The allele frequency tendencies of the chromosomes differed for each map. For the same chromosomes, predominance of both Japonica and Indica homozygotes occurred. The frequency of the heterozygotes also differed in each cross.

View larger version (79K): In this window In a new window Download PPT slide   Figure 1. Frequencies of each allele are plotted along the three genetic linkage maps, Fl1087/Dao Ren Qiao (FD), Nipponbare/Kasalath (NK), and Fl-1007/Kinandang puti (FK). For each chromosome, the left and right ends correspond to the short and long arms, respectively, of the genetic linkage maps. The frequencies of Japonica homozygous (orange diamond), Indica homozygous (green star), and heterozygous (purple square) genotypes of the individual markers are plotted at the marker positions. Transverse thin lines connect the locations of common markers. The best regression curves of each allele frequency on the chromosome are also shown as the respective color lines. The arrows denote locations of the reproductive barriers. The up and down arrows indicate the positions of gametophytic barriers that have the effect of increasing Japonica and Indica genotypes, respectively. The gametophytic barriers affecting different genders (G1 or G2 in Table 1 and Table 3) on the same chromosome are noted by different vertical arrow positions: G1, bottom row; and G2, top row. For example, at 100 cM on chromosome 1 in FD, there are two gametophytic barriers, which affect male and female gametes independently. Double-headed arrows indicate the positions of barriers that affect zygotic viability. The regions where apparent recombination frequencies were altered by flanking reproductive barriers are indicated by different colors.

To estimate the map position, type, and intensity of the reproductive barriers for each cross, multiresponse nonlinear regression analyses of allele frequencies in the F₂ population were performed for every chromosome showing deviation from Mendelian segregation ratios. Expression of the intensity of a gametophytic reproductive barrier differs from that of a zygotic reproductive barrier. The intensity of gametophytic barriers is expressed by the transmission rate of the Japonica allele, T_J, and the intensity of zygotic barriers is expressed by the viabilities of the Indica homozygote and heterozygote relative to the Japonica homozygote, V_I, V_H. The results of the regression analysis with the smallest variance are shown in Table 1 (FD gametophytic barriers), Table 2 (FD zygotic barriers), Table 3 (FK gametophytic barriers), and Table 4 (FK zygotic barriers). The expected frequencies at each reproductive barrier locus were calculated assuming that no other barriers act on the same chromosome and are listed in the tables. To compare the intensities of the gametophytic and zygotic reproductive barriers, ² values of the Mendelian segregation ratios were calculated as an index of barrier intensity, using the expected frequency, and the values are listed in Table 1 Table 2 Table 3 Table 4. However, alternative models could also explain some of the allele frequency deviation from Mendelian expectations as a slight increment in the variance. These alternative explanations of the barriers are listed in the Alternative columns in Table 1 Table 2 Table 3 Table 4. Alternative models could explain allele frequency deviation in 14 out of 28 and 12 out of 31 regions in the FD and FK maps, respectively. In the NK map, alternative models could explain 10 out of 27 regions (HARUSHIMA et al. 2001 ).

The results of the regression analysis for deviation from Mendelian segregation ratios in NK were reported previously, and allele frequencies were explained by the presence of 15 gametophytic and 18 zygotic reproductive barriers (HARUSHIMA et al. 2001 ). For comparison with the FD and FK results, the observed marker allele frequencies with the curves best fitting the regression analysis of NK are also presented in Fig 1. Arrows indicate barrier locations. Since the FD and FK populations were smaller than the NK population, the mean squares for deviations from the regression curves of FD and FK were higher than that for NK (Fig 1).

Reproductive barriers in FD:
As shown in Fig 1, the distortions of allele frequencies in FD were explained by 32 reproductive barriers consisting of 15 gametophytic (Table 1) and 17 zygotic (Table 2) barriers. Eight gametophytic barriers preferentially transmitted a Japonica allele (T_J > 50%), and 7 barriers preferentially transmitted an Indica allele (T_J < 50%). The highest and lowest Japonica transmission rates of the gametophytic barriers were 78.2% on chromosome 8 and 6.9% on chromosome 6, respectively.

In general, zygotic barriers can be classified into five types according to heterozygote viability: (1) overdominance, the heterozygote viability is higher than that of both homozygotes (V_H > 1 and V_H > V_I); (2) underdominance, the heterozygote viability is lower than that of both homozygotes (V_H < 1 and V_H < V_I); (3) Indica dominance, the heterozygote viability is equal to that of the Indica homozygote (V_H = V_I); (4) Japonica dominance, the heterozygote viability is equal to that of the Japonica homozygote (V_H = 1); and (5) codominance, the heterozygote viability is between that of the Indica and Japonica homozygotes (1 < V_H < V_I or 1 > V_H > V_I).

Five reproductive barriers showing overdominance were found on chromosomes 2, 4, 6, 8, and 11. These are indicated by an "o" in Table 2. However, the barriers at 69.9 cM on chromosome 6 and at 115 cM on chromosome 8 showed Japonica recessive weakness rather than overdominance, since the heterozygote viabilities of these two barriers were nearly the same as that of the Indica homozygotes. The heterozygote viability of the barrier at 121.4 cM on chromosome 2 was twice that of either homozygote. Allele frequencies on chromosome 11 in FD were characteristic of overdominance (Fig 1). Five underdominance barriers were observed on chromosomes 2, 4, 5, and 9, and these are indicated by a "u" in Table 2. For 4 of the 5 underdominance barriers, the viabilities of the homozygotes were similar (V_I = 1), and the heterozygote viabilities were 50–75% that of the homozygotes. The viability of the heterozygote was sometimes equal to that of either homozygote. There were four Indica dominant barriers, at 69.9 cM on chromosome 6, at 115 cM on chromosome 8, at 83.1 cM on chromosome 9, and at 71.4 cM on chromosome 10. The Japonica homozygote viability of the Indica dominant barrier on chromosome 10 was weaker than both Indica homozygote and heterozygote viabilities. In contrast, the Japonica homozygote viabilities of the Indica dominant barriers on chromosomes 6, 8, and 9 were higher than the other viabilities. There was only one Japonica dominant barrier, at 35.5 cM on chromosome 8. The Indica homozygote viability of the Japonica dominant barrier was more vigorous than the others. ² values of the Mendelian segregation ratios for each barrier revealed strong barriers at 9 of 15 gametophytic barriers and 6 of 17 zygotic barriers. The strongest barrier in FD was the zygotic barrier at 33.0 cM on chromosome 3. If this zygotic barrier induces semisterility, its expected fertility alone is 50%. Thus, the apparent fertility of the FD hybrid, 50%, would be due in large part to this zygotic reproductive barrier.

Reproductive barriers in FK:
Allele frequency distortions in FK (Fig 1) were explained by 37 reproductive barriers: 19 gametophytic (Table 3) and 18 zygotic (Table 4). Nine gametophytic barriers preferentially transmitted a Japonica allele. The highest and the lowest Japonica transmission rates of the gametophytic barriers were 85.3% on chromosome 1 and 10.1% on chromosome 6, respectively. Two zygotic barriers showed distinct overdominance, at 73.9 cM on chromosome 7 and at 100.5 cM on chromosome 8, indicated by an "o" in Table 4. The heterozygote viability of the barrier on chromosome 8 was twice that of either homozygote, whose viabilities were approximately equal. Seven zygotic reproductive barriers on chromosomes 2, 3, 5, 8, 9, 11, and 12 displayed clear underdominance, indicated by a "u" in Table 4. The heterozygote viability of the barrier on chromosome 11 was one-half that of either homozygote, and the allele frequencies of the markers on that chromosome were typical of an underdominance barrier (Fig 1). In FK, there were three Japonica dominant zygotic barriers: at 96.7 cM on chromosome 2, at 37.1 cM on chromosome 7, and at 95.0 cM on chromosome 9. Four Indica dominant zygotic barriers were found: at 82.6 cM on chromosome 1, at 95.8 cM on chromosome 4, and at 13.1 and 51.2 cM on chromosome 10. Nine of 19 gametophytic and 3 of 18 zygotic barriers had strong distortion effects on the segregation ratios. The strongest barrier in FK was the zygotic barrier at 188 cM on chromosome 1. If this barrier alone induced semisterility, the expected fertility would be 30%. However, the fertility of the FK hybrid was 50%. The deviation from Mendelian segregation ratios of the long arm of chromosome 1 could also be explained by two gametophytic barriers in the male and female instead of one zygotic barrier (Table 4). In the case of two gametophytic barriers, the expected fertility caused by the female barrier, which induces gametophytic abortion, is 50%. Therefore, it is likely that this deviation is due to two gametophytic barriers, rather than to one zygotic barrier. Further experiments are needed to confirm individual barrier functions, whether the barriers are involved in gametophytic competition, gametophytic abortion, zygotic abortion, or germination effects.

Recombination frequency between reproductive barriers:
In some regions, the observed recombination frequencies between markers varied in different populations (ANTONIO et al. 1996 ). A single reproductive barrier on a chromosome does not affect the apparent recombination frequency between markers. However, more than two barriers on a chromosome do affect the apparent recombination frequency in the region between the barriers (LORIEUX et al. 1995 ). To evaluate the effects of reproductive barriers on the observed recombination frequency, recombination frequencies in the FD and FK maps were compared to those of the NK map, with regard to the location of the reproductive barriers in these crosses. Common marker positions and regions where recombination frequencies were altered by flanking reproductive barriers are indicated in Fig 1 by thin lines and colored bars, respectively. The apparent alternation of recombination frequency by the flanking barriers is due to different transmission rates between recombinants and nonrecombinants. If two gametophytic reproductive barriers at different loci on a chromosome tended to transmit opposite genotypes, recombinants between the barriers would be transmitted preferentially, and the apparent recombination frequency between the barriers would be higher than the intrinsic frequency. In contrast, if two gametophytic barriers tended to transmit the same genotype, the apparent recombination frequency between the barriers would be lower than the intrinsic frequency. The alternation of recombination frequency by flanking barriers is the same between the barriers, because the alternation is unaffected by the location of recombination between the barriers. As shown in Fig 1, most recombination frequency differences between common markers among different populations do not seem to be correlated with the locations of the barriers. The main causes of the recombination frequency differences are not allele frequency deviations caused by reproductive barriers, but other factors that alter recombination frequencies on the respective regions in the particular crosses.

Comparison of barriers among crosses:
The deviation from the Mendelian segregation ratios of the marker allele frequencies differed in magnitude and direction among the three Japonica-Indica crosses, as shown in Fig 1. However, the numbers of reproductive barriers in the crosses were similar in the three crosses. To examine the possibility of common barrier loci in Japonica and Indica rice cultivars, the respective positions of barriers detected in FD and FK were estimated in the NK map using common markers (Fig 1). Reproductive barriers detected in the three crosses are indicated at their respective positions in the NK linkage map in Fig 2. We examined the possibility of reproductive barriers common to two of the crosses, considering the barrier position and the barrier type using alternative explanations, whether it was gametophytic or zygotic. Estimated errors of barrier positions in the regression analysis and flanking common markers were considered. We ignored differences in the intensity and direction of the allele distortion, because these differences would be due to alleles of the same reproductive barrier. Maximally, barriers in 19 regions might be common reproductive barriers in either combination of two crosses, as indicated by boxes in Fig 2. Barriers potentially common to all three crosses were found at four regions: the center of chromosome 1, 50 cM of chromosome 2, 100 cM of chromosome 4, and the long arm of chromosome 5. The number of barriers detected in each cross and the maximal number of common barriers are summarized in Table 5. The maximal probabilities of a common barrier between two crosses can be estimated by dividing the number of common barriers by the total number of different barriers in two crosses. The maximal probabilities of common barriers for two crosses were 20.4, 18.6, and 16.9% for NK and FD, NK and FK, and FD and FK, respectively. These values were similar for all pairs. At least 79 of the 102 total barriers detected in the three crosses were at different loci. Since a genetic linkage map that has an equal probability of apparent recombination frequency on the map is constructed, selection by chance in the gametophyte or zygote appears as deviation due to reproductive barriers. False barriers might randomly appear in the map and decrease the apparent probability of a common barrier. To eliminate the effects of false barriers on estimating the common barrier probability, barriers with strong intensities were compared. Nevertheless, the maximal probability of common barriers between two crosses decreased to 8%, as shown in parentheses in Table 5. No barrier was common to all three crosses. At least 36 of the 42 strong barriers detected in these crosses were at different loci.

View larger version (29K): In this window In a new window Download PPT slide   Figure 2. The locations of reproductive barriers in three Japonica-Indica crosses were coordinated on the rice genetic linkage map constructed using NK. The orange, green, and blue arrows represent reproductive barriers detected in NK, FD, and FK, respectively. The respective locations for the reproductive barriers detected in crosses other than NK were estimated using flanking markers common to the NK map. Black lines indicate common markers. Arrows are as described in the Fig 1 legend. Boxed arrows indicate the possibility of allelic barriers for various crosses.

	DISCUSSION

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

The intensive characterization of the reproductive barriers in the three Japonica-Indica crosses reported here made it possible to do the first genome-wide comparison of the barriers in differentiating populations and to discuss the evolution of the reproductive barriers. In each of the three Japonica-Indica crosses, we detected many variations in the reproductive barriers, which considered male gametophytic barriers, female gametophytic barriers, Japonica-biased barriers, Indica-biased barriers, overdominance barriers, underdominance barriers, etc. The numbers of reproductive barriers causing deviation from Mendelian segregation ratios in each cross were similar; however, most of them mapped at different loci. If reproductive barriers were formed before Japonica-Indica differentiation, many common reproductive barriers would be expected. Our observations indicate that the many different genes or elements that act as reproductive barriers in diverse rice cultivars evolved after Japonica-Indica differentiation.

If genetic variations within either Japonica or Indica varieties were large, diverse variations of the reproductive barriers in the three Japonica-Indica hybrids would be reasonable. Nevertheless, the genetic variation within both Japonica and Indica varieties appeared small. In this study, 70–80% of markers showed the same RFLP in FD and FK as in NK (ANTONIO et al. 1996 ). This means that randomly detected differences in the genomic sequences of NK are conserved with 70–80% probability in FD and FK. If the barriers detected in NK were due to a sequence difference between Nipponbare and Kasalath, each barrier should have been detected at the same locus with 70–80% probabilities in the other crosses. However, the barriers detected in NK preserved at a very low rate, 33% (11/33), in both FD and FK. For strong barriers, the barrier conservation rate decreased to 13% (2/15). A probable explanation is that the barriers in the three crosses evolved more rapidly than the other sequence changes. If the detected Japonica-Indica barrier was due to sequence differences between the Japonica and Indica varieties, the barrier need not act in a Japonica-Japonica cross or in an Indica-Indica cross. However, there are some reports of significantly distorted segregation of markers in Japonica-Japonica and Indica-Indica crosses (LIN et al. 1996 ; REDONA and MACKILL 1996 ). This suggests that some reproductive barriers also act between lines of the same type, Japonica or Indica.

Recently, rapid evolution of the genes responsible for reproductive isolation in abalone (LEE et al. 1995 ), sea urchin (METZ and PALUMBI 1996 ), Chlamydomonas (FERRIS et al. 1997 ), and Drosophila (PALOPOLI and WU 1996 ; TING et al. 1998 ) was reported. Although the gamete recognition systems for reproductive isolation between closely related species were different, rapid evolution of the genes involved appears to be a common feature (PALUMBI 1998 ; VACQUIER 1998 ). Several hypotheses to explain positive Darwinian selection in these genes for prezygotic isolation have been proposed (VACQUIER et al. 1997 ; HOWARD et al. 1998 ; PALUMBI 1998 ; RICE 1998 ; GAVRILETS 2000 ). Sexual conflict, which occurs when characteristics that enhance the reproductive success of one sex reduce the fitness of the other sex, is a major candidate for the driving force of positive selection. However, our results suggest rapid evolution takes place not only in gametophytic barriers, but also in zygotic reproductive barriers of Japonica and Indica cultivars.

What was the selective pressure behind the rapid evolution of reproductive barriers in rice cultivars? We suspect that it was the domestication of rice. The process of domestication involved repeated selection for productivity; thus the many mutations in genes promoting high seed production would have accumulated. Moreover, the domestication of rice changed the breeding system, from outcrossing to inbreeding, and fixed these mutations. The rapid evolution of reproductive barriers between the Japonica and Indica cultivars would have occurred as a by-product of the domestication of rice. Though the dynamics of the Japonica-Indica differentiation remain unknown (OKA 1988 ), a tendency toward Japonica-Indica differentiation in O. rufipogon strains that were the wild progenitors of Asian cultivated rice was reported through an analysis of allozyme variations and morphological characteristics (MORISHIMA and GADRINAB 1987 ). Analysis of reproductive barriers between wild rice strains should elucidate the effects of domestication on the evolution of reproductive barriers.

	ACKNOWLEDGMENTS

We thank H. Morishima and the editor for valuable discussion. This work was supported in part by a grant from the Ministry of Agriculture, Forestry, and Fisheries of Japan (Rice Genome Project MP-1117).

Manuscript received December 21, 2000; Accepted for publication October 8, 2001.

	LITERATURE CITED

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

ANTONIO, B. A., T. INOUE, H. KAJIYA, Y. NAGAMURA, and N. KURATA et al., 1996  Comparison of genetic distance and order of DNA markers in five populations of rice. Genome 39:946-956.

FERRIS, P. J., C. PAVLOVIC, S. FABRY, and U. W. GOODENOUGH, 1997  Rapid evolution of sex-related genes in Chlamydomonas.. Proc. Natl. Acad. Sci. USA 94:8634-8639[Abstract/Free Full Text].

GAO, B., L. E. KLEIN, R. J. BRITTEN, and E. H. DAVIDSON, 1986  Sequence of mRNA coding for bindin, a species-specific sea urchin sperm protein required for fertilization. Proc. Natl. Acad. Sci. USA 83:8634-8638[Abstract/Free Full Text].

GAVRILETS, S., 2000  Rapid evolution of reproductive barriers driven by sexual conflict. Nature 403:886-889[Medline].

HARUSHIMA, Y., M. YANO, A. SHOMURA, M. SATO, and T. SHIMANO et al., 1998  A high-density rice genetic linkage map with 2275 markers using a single F₂ population. Genetics 148:479-494[Abstract/Free Full Text].

HARUSHIMA, Y., M. NAKAGAHRA, M. YANO, T. SASAKI, and N. KURATA, 2001  A genome-wide survey of reproductive barriers in an intraspecific hybrid. Genetics 159:883-892[Abstract/Free Full Text].

HERRMANN, B. G., B. KOSCHORZ, K. WERTZ, K. J. MCLAUGHLIN, and A. KISPERT, 1999  A protein kinase encoded by the t complex responder gene causes non-mendelian inheritance. Nature 402:141-146[Medline].

HOWARD, D. J., M. REECE, P. G. GREGORY, J. CHU and M. L. CAIN, 1998 The evolution of barriers to fertilization between closely related organisms, pp. 279–288 in Endless Forms: Species and Speciation, edited by D. J. HOWARD and S. B. BERLOCHER. Oxford University Press, New York.

KURATA, N., Y. NAGAMURA, K. YAMAMOTO, Y. HARUSHIMA, and N. SUE et al., 1994  A 300 kilobase interval genetic map of rice including 883 expressed sequences. Nat. Genet. 8:365-372[Medline].

LEE, Y.-H., T. OTA, and V. D. VACQUIER, 1995  Positive selection is a general phenomenon in the evolution of abalone sperm lysin. Mol. Biol. Evol. 12:231-238[Abstract].

LIN, H.-X., H.-R. QIAN, J.-Y. ZHUANG, J. LU, and S.-K. MIN et al., 1996  RFLP mapping of QTLs for yield and related characters in rice (Oryza sativa L.). Theor. Appl. Genet. 92:920-927.

LORIEUX, M., X. PERRIER, B. GOFFINET, C. LANAUD, and D. GONZÁLEZ DE LEÓN, 1995  Maximum-likelihood models for mapping genetic markers showing segregation distortion. 2. F₂ populations. Theor. Appl. Genet. 90:81-89.

MERRILL, C., L. BAYRAKTAROGLU, A. KUSANO, and B. GANETZKY, 1999  Truncated RanGAP encoded by the segregation distorter locus of Drosophila.. Science 283:1742-1745[Abstract/Free Full Text].

METZ, E. C. and S. R. PALUMBI, 1996  Positive selection and sequence rearrangements generate extensive polymorphism in the gamete recognition protein bindin. Mol. Biol. Evol. 13:397-406[Abstract].

MORISHIMA, H., and L. U. GADRINAB, 1987 Are the Asian common wild-rices differentiated into the Indica and Japonica types?, pp. 11–20 in Crop Exploration and Utilization of Genetic Resources, edited by S.-C. HSIEH. Taichung District Agricultural Import Station, Taiwan.

OKA, H.-I., 1988 Indica-Japonica differentiation of rice cultivars, pp. 141–179 in Origin of Cultivated Rice. JSSP, Tokyo/Elsevier, Amsterdam.

PALOPOLI, M. F. and C.-I WU, 1996  Rapid evolution of a coadapted gene complex: evidence from the segregation distorter (SD) system of meiotic drive in Drosophila melanogaster.. Genetics 143:1675-1688[Abstract].

PALUMBI, S. R., 1998 Species formation and the evolution of gamete recognition loci, pp. 271–278 in Endless Forms: Species and Speciation, edited by D. J. HOWARD and S. B. BERLOCHER. Oxford University Press, New York.

REDOÑA, E. D. and D. J. MACKILL, 1996  Molecular mapping of quantitative trait loci in japonica rice. Genome 39:395-403.

RICE, W. R., 1998 Intergenomic conflict, interlocus antagonistic coevolution, and the evolution of reproductive isolation, pp. 261–270 in Endless Forms: Species and Speciation, edited by D. J. HOWARD and S. B. BERLOCHER. Oxford University Press, New York.

SWANSON, W. J. and V. D. VACQUIER, 1997  The abalone egg vitelline envelope receptor for sperm lysin is a giant multivalent molecule. Proc. Natl. Acad. Sci. USA 94:6724-6729[Abstract/Free Full Text].

TING, C.-T., S.-C. TSAUR, M.-L. WU, and C.-I WU, 1998  A rapidly evolving homeobox at the site of a hybrid sterility gene. Science 282:1501-1504[Abstract/Free Full Text].

VACQUIER, V. D., 1998  Evolution of gamete recognition proteins. Science 281:1995-1998[Abstract/Free Full Text].

VACQUIER, V. D., K. R. CARNER, and C. D. STOUT, 1990  Species-specific sequences of abalone lysin, the sperm protein that creates a hole in the egg envelope. Proc. Natl. Acad. Sci. USA 87:5792-5796[Abstract/Free Full Text].

VACQUIER, V. D., W. J. SWANSON, and Y.-H. LEE, 1997  Positive Darwinian selection on two homologous fertilization proteins: what is the selective pressure driving their divergence? J. Mol. Evol. 44(Suppl. 1):S15-S22.

This article has been cited by other articles:

				T. Nakazato, M.-K. Jung, E. A. Housworth, L. H. Rieseberg, and G. J. Gastony A Genomewide Study of Reproductive Barriers Between Allopatric Populations of a Homosporous Fern, Ceratopteris richardii Genetics, October 1, 2007; 177(2): 1141 - 1150. [Abstract] [Full Text] [PDF]

				A. L. Sweigart, L. Fishman, and J. H. Willis A Simple Genetic Incompatibility Causes Hybrid Male Sterility in Mimulus Genetics, April 1, 2006; 172(4): 2465 - 2479. [Abstract] [Full Text] [PDF]

				A. Bouck, R. Peeler, M. L. Arnold, and S. R. Wessler Genetic Mapping of Species Boundaries in Louisiana Irises Using IRRE Retrotransposon Display Markers Genetics, November 1, 2005; 171(3): 1289 - 1303. [Abstract] [Full Text] [PDF]

				J. M. Wan, Y. J. Cao, C. M. Wang, and H. Ikehashi Quantitative Trait Loci Associated with Seed Dormancy in Rice Crop Sci., February 23, 2005; 45(2): 712 - 716. [Abstract] [Full Text] [PDF]

				N. Kurata, K. Miyoshi, K.-I. Nonomura, Y. Yamazaki, and Y. Ito Rice Mutants and Genes Related to Organ Development, Morphogenesis and Physiological Traits Plant Cell Physiol., January 15, 2005; 46(1): 48 - 62. [Abstract] [Full Text] [PDF]

				L. C. Moyle and E. B. Graham Genetics of Hybrid Incompatibility Between Lycopersicon esculentum and L. hirsutum Genetics, January 1, 2005; 169(1): 355 - 373. [Abstract] [Full Text] [PDF]

				H. Kuittinen, A. A. de Haan, C. Vogl, S. Oikarinen, J. Leppala, M. Koch, T. Mitchell-Olds, C. H. Langley, and O. Savolainen Comparing the Linkage Maps of the Close Relatives Arabidopsis lyrata and A. thaliana Genetics, November 1, 2004; 168(3): 1575 - 1584. [Abstract] [Full Text] [PDF]

				K. Matsubara, Khin-Thidar, and Y. Sano A Gene Block Causing Cross-Incompatibility Hidden in Wild and Cultivated Rice Genetics, September 1, 2003; 165(1): 343 - 352. [Abstract] [Full Text] [PDF]

				N. KURATA, K.-I. NONOMURA, and Y. HARUSHIMA Rice Genome Organization: the Centromere and Genome Interactions Ann. Bot., October 1, 2002; 90(4): 427 - 435. [Abstract] [Full Text] [PDF]