The pattern of pairing that is effective for crossing over in complex B-A chromosome rearrangements in maize

Special stocks involving complex B and A doubly translocated and recombined chromosomes were utilized to study the frequency and distribution of chiasmata under constrained conditions. The studies allow comparisons of frequency of pairing effective for crossing over in segments of different length and location in chromosomes that are present in disomic and trisomic quantity. Results provide new evidence for independent initiation of effective pairing in intercalary chromosome regions and suggest sequential events in the establishment of effective pairing, some of which may depend upon synaptic extension or two-by-two prealignment. Pairing frequency may depend directly on segment length under potentially competitive conditions. Evidence was not found for heterogeneity of pairing capacity within the regions studied.     

The Development and Meiotic Behavior of Asymmetrical Isochromosomes in Wheat

To determine which segments of a chromosome arm are responsible for the initiation of chiasmate pairing in meiosis, a series of novel isochromosomes was developed in hexaploid wheat (Triticum aestivum L.). These isochromosomes are deficient for different terminal segments in the two arms. It is proposed to call them ``asymmetrical.' Meiotic metaphase I pairing of these asymmetrical isochromosomes was observed in plants with various doses of normal and deficient arms. The two arms of an asymmetrical isochromosome were bound by a chiasma in only two of the 1134 pollen mother cells analyzed. Pairing was between arms of identical length whenever such were available; otherwise, there was no pairing. However, two arms deficient for the same segment paired with a frequency similar to that of normal arms, indicating that the deficient arms retained normal capacity for pairing. Pairing of arms of different length was prevented not by the deficiency itself, but rather, by the heterozygosity for the deficiency. Whether two arms were connected via a centromere in an isochromosome or were present in two different chromosomes had no effect on pairing. This demonstrates that in the absence of homology in the distal regions of chromosome arms, even if relatively short, very long homologous segments may remain unrecognized in meiosis and will not be involved in chiasmate pairing.     

Synaptonemal complex spreading in Allium cepa and A. fistulosum

The general features and fine structure of homologous chromosome alignment and pairing have been investigated in two species of Allium (A. fistulosum and A. cepa), which have similar karyotypes but very different patterns of chiasma distribution. Although there is no support for the occurrence of a general pre-meiotic alignment of homologous chromosomes, both species show some alignment of homologues as an immediate prelude to synaptonemal complex (SC) formation. In both species pairing usually commences at sub-terminal sites and is succeeded by numerous separate intercalary initiations of pairing in interstitial and distal regions and then in proximal regions. The last parts to pair, in both species, are pericentromeric and telomeric regions. There is, therefore, no evident relationship between the sequence of pairing and chiasma distribution in these species. Regularly alternating convergences and divergences of aligned axial cores (ACs), termed multiple association sites, are frequently observed. It is proposed that these represent potential pairing initiation sites and from observations on their spatial distribution it is argued that they may be evenly distributed through most of the genome. Small spherical or ellipsoid nodules are found at association sites and between closely aligned ACs which persist in the SC segments present during zygotene, but most of them disappear abruptly at the end of zygotene. These are termed zygotene nodules (ZN) and it is proposed that they are involved in matching corresponding sites on homologous chromosomes as well as possibly having a recombinational role. Their composition, structure, mode of action and relationship to pachytene recombination nodules are at present unknown.     

The pattern of pairing that is effective for crossing over in complex B-A chromosome rearrangements in maize

The study of the mechanism of meiotic homolog pairing, approached by comparing chiasma frequencies in rearranged segments that differ in relative length and intrachromosomal location, is substantially extended here. For the first time, two kinds of evidence were found that centers specialized for alignment pairing may exist in maize chromosomes: (1) for two segments, higher than average crossover frequency per unit length was maintained when these were located in several different chromosomal positions with respect to centromere and telomere, and in fact apart from their own normal centromeres and telomeres. High crossover frequencies in these segments regardless of position are considered to reflect innate capacity for alignment pairing due to relatively strong pairing center content. (2) For a short rearranged segment, chiasma frequency was drastically reduced, and evidence suggests that all of the chiasmata found there depended upon juxtaposition made possible by the completion of the zip-up pairing process in the other arms of the translocation configuration. This short segment is thought to be essentially devoid of pairing center content. It seems possible that crossover frequency depression in short rearranged segments may usually not be due, as commonly supposed, to mechanical difficulties inherent in formation of contorted configurations, but rather to absence of pairing centers within them and the relative rarity (compared to the normal sequence situation) of enabling zip-up pairing. Evidence also indicates that pairing which leads to crossing over must frequently occur between internal translocated segments and their normal sequence counterparts in a way which cannot be dependent upon zipping-up of two-by-two pairing initiated at or near telomeres. Pairing centers in maize are probably numerous and widely dispersed, since coarse direct proportionality is found when chiasma frequency is compared for an array of segment lengths.     

Physical distribution of recombination in B-genome chromosomes of tetraploid wheat

Summary Several studies have indicated a noncorrespondence between genetic and physical distances in wheat chromosomes. To study the physical distribution of recombination, polymorphism for C-banding patterns was used to monitor recombination in 67 segments in 11 B-genome chromosome arms of Triticum turgidum. Recombination was absent in proximal regions of all chromosome arms; its frequency increased exponentially with distance from the centromere. A significant difference was observed between the distribution of recombination in physically short and physically long arms. In physically short arms, recombination was almost exclusively concentrated in distal segments and only those regions were represented in their genetic maps. In physically long arms, while a majority of the genetic distance was again based upon recombination in distal chromosome segments, some interstitial recombination was observed. Consequently, these regions also contributed to the genetic maps. Such a pattern of recombination, skewed toward terminal segments of chromosomes, is probably a result of telomeric pairing initiation and strong positive chiasma interference. Interference averaged 0.81 in 35 pairs of adjacent segments and 0.57 across the entire recombining portions of chromosome arms. The total genetic map lengths of the arms corresponded closely to those expected on the basis of their metaphase-I chiasma frequencies. As a consequence of this uneven distribution of recombination there can be a 153-fold difference (or more) in the number of DNA base pairs per unit (centiMorgan) of genetic length.     

The mechanism of meiotic homologue pairing

Homologous chromosome pairing involves the moving together of matching chromosomes or chromosome segments across substantial distances within a nucleus. Although the time in the life cycle of initial association of homologues varies among organisms, it may well be that similar underlying mechanisms for its occurrence prevail throughout sexually reproducing eukaryotes. The means by which pairing its accomplished is in no case understood. In the apparent absence of a long range specific force of attraction, simple partial models have been proposed which relay for the most part upon interactions of chromosome ends (telomeres) with specialized portions of the nuclear envelope. While such interactions, as well as the persistence of chromosome orientation established by mitotic anaphase poleward movement of centromere regions, may provide in many cases for closer than random positioning of some parts of homologues, the distances remaining to be traversed are still long range in physical-chemical terms. Also, the specific pairing observed in some kinds of rearranged segments is not facilitated by such circumstances, even if synapsis is initiated at available homologous telomere pairs and proceeds to completion by a "zip-up" process. A unified, more complex model is considered which is designed to accommodate the various relevant findings. It invokes the interaction of intranuclear structures with intercalary and/or terminal chromosomal pairing sites, e.g. filamentous structures which specifically bind to these, and a contractile system involving proteins such as actin and myosin to draw homologues together.     

Dynamics of rye chromosome 1R regions with high or low crossover frequency in homology search and synapsis development

In many organisms, homologous pairing and synapsis depend on the meiotic recombination machinery that repairs double-strand DNA breaks (DSBs) produced at the onset of meiosis. The culmination of recombination via crossover gives rise to chiasmata, which locate distally in many plant species such as rye, Secale cereale. Although, synapsis initiates close to the chromosome ends, a direct effect of regions with high crossover frequency on partner identification and synapsis initiation has not been demonstrated. Here, we analyze the dynamics of distal and proximal regions of a rye chromosome introgressed into wheat to define their role on meiotic homology search and synapsis. We have used lines with a pair of two-armed chromosome 1R of rye, or a pair of telocentrics of its long arm (1RL), which were homozygous for the standard 1RL structure, homozygous for an inversion of 1RL that changes chiasma location from distal to proximal, or heterozygous for the inversion. Physical mapping of recombination produced in the ditelocentric heterozygote (1RL/1RL(inv)) showed that 70% of crossovers in the arm were confined to a terminal segment representing 10% of the 1RL length. The dynamics of the arms 1RL and 1RL(inv) during zygotene demonstrates that crossover-rich regions are more active in recognizing the homologous partner and developing synapsis than crossover-poor regions. When the crossover-rich regions are positioned in the vicinity of chromosome ends, their association is facilitated by telomere clustering; when they are positioned centrally in one of the two-armed chromosomes and distally in the homolog, their association is probably derived from chromosome elongation. On the other hand, chromosome movements that disassemble the bouquet may facilitate chromosome pairing correction by dissolution of improper chromosome associations. Taken together, these data support that repair of DSBs via crossover is essential in both the search of the homologous partner and consolidation of homologous synapsis.     

Genetic and physical mapping of homoeologous recombination points involving wheat chromosome 2B and rye chromosome 2R.

Wide hybrids have been used in generating genetic maps of many plant species. In this study, genetic and physical mapping was performed on ph1b-induced recombinants of rye chromosome 2R in wheat (Triticum aestivum L.). All recombinants were single breakpoint translocations. Recombination 2RS-2BS was absent from the terminal and the pericentric regions and was distributed randomly along an intercalary segment covering approximately 65% of the arm's length. Such a distribution probably resulted from structural differences at the telomeres of 2RS and wheat 2BS arm that disrupted telomeric initiation of pairing. Recombination 2RL-2BL was confined to the terminal 25% of the arm's length. A genetic map of homoeologous recombination 2R-2B was generated using relative recombination frequencies and aligned with maps of chromosomes 2B and 2R based on homologous recombination. The alignment of the short arms showed a shift of homoeologous recombination toward the centromere. On the long arms, the distribution of homoeologous recombination was the same as that of homologous recombination in the distal halves of the maps, but the absence of multiple crossovers in homoeologous recombination eliminated the proximal half of the map. The results confirm that homoeologous recombination in wheat is based on single exchanges per arm, indicate that the distribution of these single homoeologous exchanges is similar to the distribution of the first (distal) crossovers in homologues, and suggest that successive crossovers in an arm generate specific portions of genetic maps. A difference in the distribution of recombination between the short and long arms indicates that the distal crossover localization in wheat is not dictated by a restricted distribution of DNA sequences capable of recombination but by the pattern of pairing initiation, and that can be affected by structural differences. Restriction of homoeologous recombination to single crossovers in the distal part of the genetic map complicates chromosome engineering efforts targeting genes in the proximal map regions.     

The initiation of homologous chromosome synapsis in mouse fetal oocytes is not directly driven by centromere and telomere clustering in the bouquet

We investigated the behaviour of centromeres and distal telomeres during the initial phases of female meiosis in mice. In particular, we wished to determine whether clustering of centromeres and telomeres (bouquet formation) played the same crucial role in homologous chromosome pairing in female meiosis as it does in the male. We found that synapsis (intimate homologous chromosome pairing) is most frequently initiated in the interstitial regions of homologous chromosomes, apparently ahead of the distal regions. The proximal ends of the chromosomes appear to be disfavoured for synaptic initiation. Moreover, initiation of synapsis occurred in oocytes that showed little or no evidence of bouquet formation. A bouquet was present in a substantial proportion of cells at mid to late zygotene, and was still present in some pachytene oocytes. This pattern of bouquet formation and pairing initiation is in stark contrast to that previously described in the male mouse. We propose that although dynamic movements of centromeres and telomeres to form clusters may facilitate alignment of homologues or homologous chromosome segments during zygotene, in the female mouse positional control of synaptic initiation is dependent on some other mechanism.     

Identification and designation of telocentric chromosomes in barley by means of Giemsa N-banding technique

Summary Seven complete chromosomes and nine telocentric chromosomes in telotrisomics of barley (Hordeum vulgare L.) were identified and designated by an improved Giemsa N-banding technique. Karyotype analysis and Giemsa N-banding patterns of complete and telocentric chromosomes at somatic late prophase, prometaphase and metaphase have shown the following results: Chromosome 1 is a median chromosome with a long arm (Telo 1L) carrying a centromeric band, while short arm (Telo 1S) has a centromeric band and two intercalary bands. Chromosome 2 is the longest in the barley chromosome complement. Both arms show a centromeric band, an intercalary band and two faint dots on each chromatid at middle to distal regions. The banding pattern of Telo 2L (a centromeric and an intercalary band) and Telo 2S (a centromeric, two intercalary and a terminal band) corresponded to the banding pattern of the long and short arm of chromosome 2. Chromosome 3 is a submedian chromosome and its long arm is the second longest in the barley chromosome complement. Telo 3L has a centromeric (fainter than Telo 3S) and an intercalary band. It also shows a faint dot on each chromatid at distal region. Telo 3S shows a dark centromeric band only. Chromosome 4 is the most heavily banded one in barley chromosome complement. Both arms showed a dark centromeric band. Three dark intercalary bands and faint telomeric dot were observed in the long arm (4L), while two dark intercalary bands in the short arm (4S) were arranged very close to each other and appeared as a single large band in metaphase chromosomes. A faint dot was observed in each chromatid at the distal region in the 4S. Chromosome 5 is the smallest chromosome, which carries a centromeric band and an intercalary band on the long arm. Telo 5L, with a faint centromeric band and an intercalary band, is similar to the long arm. Chromosomes 6 and 7 are satellited chromosomes showing mainly centromeric bands. Telo 6S is identical to the short arm of chromosome 6 with a centromeric band. Telo 3L and Telo 4L were previously designated as Telo 3S and Telo 4S based on the genetic/linkage analysis. However, from the Giemsa banding pattern it is evident that these telocentric chromosomes are not correctly identified and the linkage map for chromosome 3 and 4 should be reversed. One out of ten triple 2S plants studied showed about 50% deficiency in the distal portion of the short arm. Telo 4L also showed a deletion of the distal euchromatic region of the long arm. This deletion (32%) may complicate genetic analysis, as genes located on the deficient segment would show a disomic ratio. It has been clearly demonstrated that the telocentric chromosomes of barley carry half of the centromere. Banding pattern polymorphism was attributed, at least partly, to the mitotic stages and differences in techniques.Contribution from the Department of Agronomy and published with the approval of the Director of the Colorado State University Experiment Station as Scientific Series Paper No. 2730. This research was supported in part by the USDA/SEA Competitive Research Grant 5901-0410-9-0334-0, USDA/ SEA-CSU Cooperative Research Grant 12-14-5001-265 and Colorado State University Hatch Project. This paper was presented partly at the Fourth International Barley Genetics Symposium, Edinburgh, Scotland, July 22–29, 1981     

DNA synthesis in the neo-X neo-Y sex determination system of Dichroplus bergi (Orthoptera: Acrididae)

DNA replication in the neo-X neo-Y sex determining system was studied by means of tritiated thymidine and autoradiography. Asynchronous replication was found in the X arm of the neo-X and the long arm of the neo-Y. In addition, striking asynchrony was also found for short isopycnotic homologous regions at the distal end of the autosmal arm of neo-X and the short arm of neo-Y to which pairing during meiosis is restricted. These short regions are asynchronous with respect to the heterochromatic segments as well as to the remaining proximal region of the autosomal euchromatic arm of neo-X. This difference in replication pattern within the same chromosome arm may be related to a differentiation between regions which are homozygous in both sexes and regions which are hemizygous in males.This work was supported by U.S. Atomic Energy Commission Contract N AT (30-1) 3517 to Prof. F. A. Saez.     

The quantitative analysis of chromosome pairing and chiasma formation based on the relative frequencies of M I configurations

In autotetraploids, chromosome pairing may be in the form of quadrivalents or bivalent pairs. Whether or not the quadrivalents are maintained until first meiotic metaphase depends on the formation of chiasmata. The relative frequencies of M I configurations thus contain information both on pairing and on chiasma formation. With distal chiasma localisation six configurations can be recognised and their relative frequencies determined: ring quadrivalents, chain quadrivalents, trivalents (with univalent), ring bivalents, open (rod) bivalents, univalent pairs. These represent five degrees of freedom permitting five parameters to be estimated: the frequency (f) of quadrivalent pairing; the frequencies of chiasmate association of the two ends (arms in metacentrics), a′, b′, after quadrivalent pairing, and a, b after bivalent pairing. — The appropriate formulae have been derived and applied to observations on Tradescantia virginiana (4n=24) which has pronounced distal chiasma localisation. Slight modifications make the model applicable to autotetraploids with interstitial in addition to distal chiasmata. In T. virginiana, chromosome pairing appeared to be random between homologues (65.8% quadrivalent pairing; 55.4% observed at M I). After quadrivalent pairing chiasmate association is frequent in the “average long” arm (95.0%) and much less so in the other arm (60.5%). This is attributed to partner exchange. After bivalent pairing chiasma frequencies are still different for the two arms (93.8% and 83.5% association respectively) but much less pronounced. Various complications are discussed.     

Heterochromatic banding patterns in Allium

The karyotypes of nine Allium species of the paniculatum group have been analysed with fluorochromes and C-banding techniques. All the species possess reduced as well as enhanced fluorescence bands which are also differentiated by C-banding and which correspond to constitutive heterochromatin. Heterochromatic segments are located in distal and intercalary positions leaving sizeable procentric regions devoid of bands. These features are indicative of a close relationship between the species of this group. However, the proportion of heterochromatin as a percentage of total chromosome length varies from about 30% to 10–15%.     

Chromosome pairing in maize

This report summarizes our observations at pachytene on opposite-arms intercrosses between stocks of interchanges that involve chromosomes 1 and 5 in maize.—Pairing does not begin at the centromeres in these intercrosses.—We propose a model which assumes different probability values along each chromosome arm for the initial or primary site of pairing. Observations on the frequencies of the different types of configurations at pachytene were used to estimate probability values which satisfactorily fit the data.—There is a relatively low probability (of the order of.1 to.3) for the initial pairing to be in a short terminal segment (about.1 of the arm length). Initial pairing in the one or two short segments adjacent to the tip segment is much higher. Initial pairing is much lower in segments successively closer to the middles of the chromosome arms, and then zero or nearly zero in the proximal half of the arm. This means that the initial pairing may fail occasionally even in a relatively long interchanged segment and produce a T-shaped (3-armed) configuration.—After the initial pairing has occurred, the average probability that a secondary site of pairing is adjacent to the centromere in a segment.3 to.4 the length of an arm is low (.13, ranging from.02 to.29).—We can predict that in an intercross in which both breakpoints in both parental interchanges are far out on the chromosomes, "pairs" will be formed with nonhomologous ends (homologous differential segments paired). In these pairing could have begun at any point in the interstitial segments, but not likely in segments close to the centromeres.—Multiple secondary sites which vary in time or in order of pairing will explain the variation in position of the cross-shaped pachytene configuration in interchange heterozygotes.—The observed configuration in any one cell is the result of a particular combination of pairing events at the various sites. This is a very different concept of pairing from previous interpretations which described it as a result of zipper-like action, and the variation in position of the pachytene cross-configuration as the result of "shifts" in position.—Our cytogenetic results and their interpretation are in close agreement with reports on chromosome ultrastructure and molecular events in the early stages of meiosis, i.e. the attachment of chromosome ends to the nuclear membrane, the manner in which synaptonemal complexes develop, and the regions of DNA whose replication is delayed until zygonema.     

Evidence of natural selection to maintain a functional domain outside of the 'core' in a large subclass of group I introns

Comparison of three closely-related, homologous Group I introns reveals conservation of RNA secondary structure and some primary sequence outside of the characteristic Group I core structure. Further examination of forty Group I introns showed that all can be placed into one of two categories based on the length of the "loop L5" region (subtended by the base-paired sequences P and Q): short (21 to 38 bases) or long (59 to 295 bases). Despite the large variation in size and sequence, all nineteen of the long L5 introns share a common structure whose features include an adenine-rich bulge at a fixed distance from the P-Q pairing. This bulge is flanked by base-paired regions of greater than or equal to 6 base pairs on the core-proximal side and greater than or equal to 3 base pairs on the distal side. In the core-proximal helix there are a large number and high proportion of deviations from the consensus sequence that maintain base-pairing. These naturally-occurring compensatory base substitutions provide compelling phylogenetic support for the existence of this pairing and indicate that the conserved structure has a function in vivo.     

Pachytene pairing and metaphase I configurations in a tetraploid somatic Lycopersicon esculentum x L. peruvianum hybrid.

In the tetraploid somatic hybrid between the diploid Lycopersicon species L. esculentum (tomato) and L. peruvianum, synaptonemal complexes formed quadrivalents in 73 of the 120 sets of four chromosomes (60.8%) in 10 cells studied in detail at pachytene. Of these, 43 had one pairing partner exchange, 22 had two, and 8 had three, very close to a Poisson distribution. The points of pairing partner exchange were concentrated at the middle of the two arms. The frequency per arm corresponded with physical arm length. There was a sharp drop around the centromere, and pericentric heterochromatin had a slightly lower probability of being involved in pairing partner exchange than euchromatin. The chromosomes align before pairing and there are several points of pairing initiation, with concentrations at or near the ends and the centromere. From zygotene to late pachytene the quadrivalent frequency decreased considerably. At late pachytene it was lower than expected with the observed high frequency of pairing partner exchange. Pairing affinity between species was only slightly lower than affinity within species, in spite of considerable genetic differentiation. The frequency of recombination nodules increased from early to late zygotene and then decreased strongly to full pachytene. There is a highly significant negative correlation between percent pairing and SC length. At metaphase I the frequency of quadrivalents was 0.444, and branched quadrivalents were rare, probably caused by interference and restriction of chiasma formation to distal euchromatin. Metaphase I quadrivalent frequency is a relatively good indication of pairing affinity in this material.     

Biochemistry of meiosis.

The process of meiosis in Lilium falls into four physiological stages - prezygotene, zygotene, pachytene, and post-pachytene. Each of these stages has distinctive metabolic characteristics. Commitment to meiosis occurs during the prezygotene interval at about the time when S-phase replication is completed. The activities following commitment are essential to synapsis inasmuch as perturbations of cells during that interval have subsequent effects on synapsis and crossing over. Just before the initiation of synapsis, a distinctive lipoprotein complex appears in the nucleus. The complex most probably functions in the process of pairing. Zygotene is marked by the delayed replication of specific intercalary segments of chromosomal DNA (Z-DNA), the replication being a necessary condition for ongoing synapsis. The replication occurs in the lipoprotein complex in the presence of a reassociation protein (r-protein). Z-DNA segments would appear to have other meiotic functions inasmuch as the replicated segments remain unligated to the body of chromosomal DNA until the beginning of chromosome disjunction. The pachytene interval is marked by an activation of endonucleolytic activity. The enzyme produces single-stranded nicks in the DNA at specific loci. These loci consist of moderately repeated segments; about 100-200 base pairs long. Extracellular agents, such as radiation, cause random nicking regardless of the meiotic stage at which they are applied. Localized nicking and repair are thus unique features of meiosis. The temporal segregation of metabolic activities concerned with pairing and crossing over and their operation in special chromosome regions constitute the most prominent features of the biochemical events associated with meiosis.     

Mate Familiarity Affects Pairing Behaviour in a Long‐Term Monogamous Lizard: Evidence from Detailed Bio‐Logging and a 31‐Year Field Study

Long‐term monogamy is most prevalent in birds but is also found in lizards. We combined a 31‐year field study of the long‐lived, monogamous Australian sleepy lizard, Tiliqua rugosa, with continuous behavioural observations through GPS data logging, in 1 yr, to investigate the duration of pair bonds, rates of partner change and whether either the reproductive performance hypothesis or the mate familiarity hypothesis could explain this remarkable long‐term monogamy. The reproductive performance hypothesis predicts higher reproductive success in more experienced parents, whereas the mate familiarity hypothesis suggests that effects of partner familiarity select for partner retention and long‐term monogamy. Rates of partner change were below 34% over a 5‐yr period and most sleepy lizards formed long‐term pair bonds: 31 partnerships lasted for more than 15 yr, 110 for more than 10 yr, and the recorded maximum was 27 yr (ongoing). In the year when we conducted detailed observations, familiar pairs mated significantly earlier than unfamiliar pairs. Previous pairing experience (total number of years paired with previous partners) had no significant effect. Early mating often equates to higher reproductive success, and we infer that is the case in sleepy lizards. Early mating of familiar pairs was not due to better body condition. We propose two suggestions about the proximate mechanisms that may allow familiar pair partners to mate earlier than unfamiliar partners. First, they may have improved coordination of their reproductive sexual cycles to reach receptivity earlier and thereby maximise fertilisation success. Second, they may forage more efficiently, benefiting from effective information transfer and/or cooperative predator detection. Those ideas need empirical testing in the future. Regardless of the mechanism, our observations of sleepy lizard pairing behaviour support the mate familiarity hypothesis, but not the reproductive performance hypothesis, as an explanation for its long‐term monogamous mating system.     

Further studies on pachytene pairing failure in maize

Summary Pairing failure in pachytene chromosomes was studied against a genetically constant background. Very significant negative correlations were found between total chromosome length per cell on the one hand and number of terminal pairing failures and their total extent (per cent length) on the other, and between number of terminal failures and their average extent (per cent length). No significant correlation was found, however, between total chromosome length per cell and the average extent (per cent length) of pairing failure. If the pairing failures are dissociations increasing in extent and number during the pachytene stage studied, the simplest reconciliation of the results requires that the average rate of their extension be roughly proportional to total chromosome length or that certain constants be related to each other as described. An alternate interpretation, that the pairing failures were present before pachytene and that chromosomes are shortening faster in cells containing them, is favored by the findings that heterogeneity is low between anthers in number of failures per cell and that no significant correlation was found between anthers in total chromosome length and number of pairing failures. The possibility is also discussed that the pairing failures are a complex combination of both initial synaptic failure and later dissociation.Distributions of chromosomes containing pairing failure at both ends and of those containing both terminal and intercalary pairing failures generally follow expectations of randomness.This work was supported by National Institute of Health Grant Number RA-6492 and by National Science Foundation Grant Number G 7068.     

Meiotic Chromosome Pairing in Triploid and Tetraploid Saccharomyces Cerevisiae

Meiotic chromosome pairing in isogenic triploid and tetraploid strains of yeast and the consequences of polyploidy on meiotic chromosome segregation are studied. Synaptonemal complex formation at pachytene was found to be different in the triploid and in the tetraploid. In the triploid, triple-synapsis, that is, the connection of three homologues at a given site, is common. It can even extend all the way along the chromosomes. In the tetraploid, homologous chromosomes mostly come in pairs of synapsed bivalents. Multiple synapsis, that is, synapsis of more than two homologues in one and the same region, was virtually absent in the tetraploid. About five quadrivalents per cell occurred due to the switching of pairing partners. From the frequency of pairing partner switches it can be deduced that in most chromosomes synapsis is initiated primarily at one end, occasionally at both ends and rarely at an additional intercalary position. In contrast to a considerably reduced spore viability (~40%) in the triploid, spore viability is only mildly affected in the tetraploid. The good spore viability is presumably due to the low frequency of quadrivalents and to the highly regular 2:2 segregation of the few quadrivalents that do occur. Occasionally, however, quadrivalents appear to be subject to 3:1 nondisjunction that leads to spore death in the second generation.