Table 5 Glossary of terms
TermDefinition
Acentric fragmentA chromosome fragment that does not contain a centromere
Acentriolar spindleA spindle lacking centrioles, most commonly found during meiosis I in oocytes; also known as an anastral spindle
Achiasmate chromosomeA chromosome that does not form a chiasma (CO)
Balancer chromosomeA chromosome that is multiply inverted and/or rearranged in comparison to the normal chromosome and that, when heterozygous with a normal-sequence chromosome, suppresses exchange and/or prevents the recovery of recombinant products; typically carries both a dominant visible marker and a homozygous lethal or sterile mutation
BiorientationThe alignment of homologous chromosomes toward opposite spindle poles during meiosis I
BivalentA pair of homologous chromosomes held together by at least one chiasma
Class I COThe predominant type of exchange event whose placement is influenced by interference and the centromere effect
Class II COA CO that is placed in a manner independent of other COs or the centromere
Central element (CE)As observed by electron microscopy, the electron-dense region of the SC that lies within the CR, midway between the lateral elements
Central region (CR)The portion of the SC between the lateral elements that encompasses the space between two homologous chromosomes; includes the transverse filament and CE proteins
Centromere clusteringThe association of the paired homologous centromeres of one chromosome with the paired centromeres of other, nonhomologous chromosomes
Centromere effectThe strong suppression of exchange events in proximity to the centromere
Centromere pairingThe association, mediated by SC proteins, of the centromeres from two homologous chromosomes
Chiasma (pl. chiasmata)A mature CO that physically links two nonsister chromatids; required for accurate meiotic chromosome segregation
Crossing overThe process that exchanges genetic material between homologous chromosomes, leading to the formation of chiasmata and recombinant chromosomes
Crossover (CO)Event marking the location in the DNA where two nonsister chromatids exchange their genetic material; the physical outcome of the process of crossing over
CO assuranceThe observed phenomenon where each chromosome is guaranteed at least one CO during meiosis
DiakinesisIn most organisms, the last phase of meiotic prophase I, during which individual chromosomes further condense in preparation for germinal vesicle breakdown and spindle assembly; Drosophila lacks a canonical diakinesis
Dicentric bridgeAn aberrant chromosome with two centromeres that is pulled apart or broken during segregation, resulting in two unstable chromosomes
DiploteneIn most organisms, the phase of meiotic prophase I in which the SC disassembles and chromosomes become individualized; Drosophila lacks a canonical diplotene
Distributive systemTerm applied to the mechanism for properly segregating chromosomes that failed to undergo exchange; denotes two very separate processes: the mechanism that ensures the segregation of nonexchange homologs and a poorly understood process that can segregate nonhomologous chromosomes
Double-strand break (DSB)A break that involves both of the Watson and Crick strands of a DNA molecule
E0 tetradA tetrad having no CO events; the fraction of E0 tetrads is used to calculate the likelihood that a pair of homologous chromosomes did not undergo crossing over
Equational exceptionExceptional progeny resulting from nondisjunction at meiosis II
EuchromatinThe more lightly-condensed portion of a chromosome that contains the majority of an organism’s genes
Gene conversionThe nonreciprocal exchange of a small amount of genetic information from one homologous chromosome to the other, resulting in a 3:1 allele ratio at that locus; can occur in association with a CO or independently of crossing over
GermariumThe structure at the tip of the ovary where egg chambers are formed and meiosis is initiated
Germinal vesicle breakdown (GVBD)The breakdown of the nuclear envelope at the end of stage 12 of oocyte development
HeterochromatinTightly condensed chromatin that contains few genes and often contains repetitive DNA sequences
Heterologous segregationThe segregation of nonhomologous chromosomes away from one another
Interchromosomal effectThe phenomenon whereby heterozygosity for chromosome aberrations, especially balancer chromosomes, suppresses crossing over between those homologs but increases crossing over on the other unbalanced chromosomes
InterferenceThe phenomenon in which a CO in one interval decreases the likelihood of a CO in an adjacent interval
KaryosomeThe condensed oocyte nucleus that forms in midprophase
Lateral element (LE)The outermost components of the SC that run along the axis of two homologous chromosomes
MarkerA mutation that produces an innocuous, easily identifiable visible phenotype, such as eye color, wing attribute, or bristle quality
Matroclinous femalesExceptional progeny derived from XX oocytes, also known as diplo-X exceptions
Meiotic mutantA stock bearing a mutation in a gene involved in a meiotic process
MissegregationSee Nondisjunction
Noncrossover (NCO) eventA gene conversion that is not associated with a CO event
NondisjunctionThe aberrant segregation of chromosomes during meiosis, such that both homologs (meiosis I) or sister chromatids (meiosis II) go to the same daughter nucleus
Nurse cellsPolyploid, interconnected support cells that produce proteins and RNAs needed by the developing oocyte
OvarioleOne of ∼16 tubules in each Drosophila ovary that contains a germarium and developing egg chambers connected sequentially from stage 1 through stage 14
Paracentric inversionA chromosome inversion that does not include the centromere
PachytenePhase of meiotic prophase I during which paired chromosomes recombine and full-length SC is established; in flies, this begins in region 2A in the germarium
Patroclinous malesExceptional progeny derived from oocytes lacking an X; also known as nullo-X exceptions
Pericentric heterochromatinRecombinationally inert portion of a chromosome located near the centromere
Pericentric inversionA chromosome inversion that encompasses the centromere
Polar ejection forceA force that pushes chromosomes away from the spindle poles and toward the central spindle during prometaphase
Prometaphase IAn intermediate period between prophase and metaphase I during which the nuclear envelope breaks down, the bipolar spindle elongates, and chromosomes undergo dynamic movements on the spindle
Recombination hotspotA location or particular DNA sequence in the genome that exhibits a higher rate of meiotic DSB formation and/or recombination than expected by chance
Recombination nodule (RN)Protein structure associated with the SC that mediates CO/chiasma formation
Reductional exceptionExceptional progeny resulting from nondisjunction at meiosis I
Sister chromatid exchangeMeiotic exchange that occurs between sister chromatids rather than homologous chromosomes
SynapsisThe full-length alignment of homologous chromosomes by the SC in preparation for recombination
Transverse filament (TF)Coiled-coil-containing SC protein that functions to span the distance between the two lateral elements
TriploidAn individual with three sets of each chromosome
Synaptonemal complex (SC)The proteinaceous, zipper-like structure that connects homologous chromosomes during prophase
VitellariumThe posterior portion of an ovariole that contains oocytes in developmental stages 2–14
ZygotenePhase of meiotic prophase I during which homologous chromosomes begin to pair and synapse; characterized in flies by the observation of short patches of synaptonemal complex in early region 2A of the germarium