Effect of triploid fitness on the coexistence of diploids and tetraploids

The conditions for the coexistence of diploids, triploids and tetraploids in a single population were investigated with a deterministic model under the assumptions that diploids might produce 2 n gametes, and that triploids had a lower fitness than other cytotypes and generated equal proportions of haploid and diploid gametes. When diploids produced only haploid gametes, the dynamics of the cytotypes were similar to that of heterozygote disadvantage with two alleles at a single locus, with triploids being equivalent to the heterozygotes. Production of 2 n gametes by diploids increased the pool of diploid gametes and created a stable equilibrium involving a majority of diploids and a minority of polyploids. When the fitness of tetraploids was equal to or higher than that of diploids, increased triploid fitness decreased the threshold of 2 n gametes necessary to deterministically fix tetraploids in the population. Conversely, when tetraploids were less fit than diploids, the rate of 2 n gamete production leading to the exclusion of diploids first decreases and then increased with increasing triploid fitness. Triploids are repeatedly found in diploid-tetraploid hybridizations and are rarely totally sterile. They might play a determinant role in the future of multiple cytotype populations. The effect of triploids depends on the relative fitness of diploids and tetraploids and is also a function of their fitness.     

Chromosome pairing and chiasma formation in autopolyploids of different Lathyrus species.

Chromosome pairing and chiasma formation were studied in natural and induced tetraploids (2n = 28) of Lathyrus odoratus (induced), Lathyrus pratensis (natural and induced), Lathyrus sativus (induced), and Lathyrus venosus (natural), as well as in triploids of L. pratensis and diploids of L. odoratus, L. pratensis, and L. sativus. All natural tetraploids appeared to be autotetraploids and their meiotic metaphase I behaviour was very similar to that of the induced autotetraploids, with average numbers of pairing partner switches exceeding 4 or even 5. Multivalent frequencies were high, but the numbers of chiasmata were not much higher than necessary to maintain the configurations. Interstitial chiasmata were common, but not predominant. Fertility was reduced, but sufficient for predominantly vegetatively reproducing species. The triploids of L. pratensis had an even higher multivalent frequency than the tetraploids, but still produced some viable progeny at or close to the tetraploid level, suggesting that in mixed populations of diploids and tetraploids, triploids can contribute to gene flow between the ploidy levels. There was no significant correlation between chiasma frequency and ring bivalent frequency in the diploids and multivalent frequency in the corresponding tetraploids. In the tetraploids, chiasma frequency and multivalent frequency were negatively correlated.     

Meiotic chromosome pairing behaviour of natural tetraploids and induced autotetraploids of Actinidia chinensis

Key message

Non-preferential chromosome pairing was identified in tetraploid Actinidia chinensis and a higher mean multivalent frequency in pollen mother cells was found in colchine-induced tetraploids of A. chinensis compared with naturally occurring tetraploids.

Abstract

Diploid and tetraploid Actinidia chinensis are used for the development of kiwifruit cultivars. Diploid germplasm can be exploited in a tetraploid breeding programme via unreduced (2n) gametes and chemical-induced chromosome doubling of diploid cultivars and selections. Meiotic chromosome behaviour in diploid A. chinensis ‘Hort16A’ and colchicine-induced tetraploids from ‘Hort16A’ was analysed and compared with that in a diploid male and tetraploid males of A. chinensis raised from seeds sourced from the wild in China. Both naturally occurring and induced tetraploids formed multivalents, but colchicine-induced tetraploids showed a higher mean multivalent frequency in the pollen mother cells. Lagging chromosomes at anaphase I and II were observed at low frequencies in the colchicine-induced tetraploids. To investigate whether preferential or non-preferential chromosome pairing occurs in tetraploid A. chinensis, the inheritance of microsatellite alleles was analysed in the tetraploid progeny of crosses between A. chinensis (4x) and A. arguta (4x). The frequencies of inherited microsatellite allelic combinations in the hybrids suggested that non-preferential chromosome pairing had occurred in the tetraploid A. chinensis parent.     

Chromosome pairing in the allotetraploid Aegilops biuncialis and a triploid intergeneric hybrid.

Chromosome pairing behaviour of the natural allotetraploid Aegilops biuncialis (genome UUMM) and a triploid hybrid Ae. biuncialis x Secale cereale (genome UMR) was analyzed by electron microscopy in surface-spread prophase I nuclei. Synaptonemal-complex analysis at zygotene and pachytene revealed that synapsis in the allotetraploid was mostly between homologous chromosomes, although a few quadrivalents were also formed. Only homologous bivalents were observed at metaphase I. In contrast, homoeologous and heterologous chromosome associations were common at prophase I and metaphase I of the triploid hybrid. It is concluded that the mechanism controlling bivalent formation in Ae. biuncialis acts mainly at zygotene by restricting pairing to homologous chromosomes, but also acts at pachytene by preventing chiasma formation in the homoeologous associations. In the hybrid the mechanism fails at both stages. Key words : Aegilops biuncialis, allotetraploid, intergeneric hybrid, pairing control, synaptonemal complex.     

Histocompatibility analyses in tetraploids induced from clonal triploid crucian carp and in gynogenetic diploid goldfish

Histocompatibility analyses in goldfish were performed using the tetraploid goldfish-crucian carp hybrid and the first generation of gynogenetic diploid (GD₁) goldfish. Tetraploids were obtained by crossing clonal triploid crucian carp with goldfish. GD₁ goldfish were produced by the suppression of the second meiotic division. Tetraploid scale grafts on triploid clone members evoked an acute rejection in 4–6 days, whereas the reverse transplants were accepted or rejected chronically. Reciprocal grafting between tetraploids showed subacute rejection in 10–12 days, although some fish showed chronic rejection in 20–30 days. On the other hand, scale grafts reciprocally exchanged among triploids were intact even 3 months after grafting, although some of them showed a unidirectional rejection pattern. Furthermore, allograft rejection among gynogens occurred between 5 and 20 days, whereas all the scale allografts between members of control siblings were rejected within 9 days. In addition, neither accelerated acute rejection nor acceptance of allografts was observed in grafts exchanged among GD₁ goldfish. These results suggest that single doses of histocompatibility alleles are effective in eliciting acute rejection, and each of the fourth haploid set of chromosomes originating from paternal goldfish might share the same histocompatibility antigens to a large extent. This experiment also indicates that the genecentromere recombination rate is quite high with respect to the histocompatibility loci in this species.     

The genetics of meiotic chromosome pairing in Lolium temulentum x Lolium perenne tetraploids

Summary The degree of preferential pairing of homologous chromosomes was estimated in a series of tetraploid hybrids of Lolium temulentum x Lolium perenne by means of cytological and genetic analyses. The correlations between the frequency of bivalents at first metaphase of meiosis in the hybrid tetraploids and the degree of preferential pairing calculated from the segregation pattern of isozyme alleles in a test cross was extremely high. The results showed clearly that suppression of heterogenetic pairing in these Lolium tetraploids is achieved by a genetic system involving the A chromosomes as well as the B chromosome system which has been known for some time. Certain similarities with the genetic system controlling pairing in polyploid wheats are discussed.     

The chromosomal location of rye AFLP bands

23 AFLP bands were assigned to different rye chromosomes by means of two different sets of wheat-rye addition lines. Only one AFLP band could be assigned to 4R, and no specific AFLPs were found on the 5R chromosome. Only one AFLP band was explicitly assigned to 4R, and no specific AFLPs were found on the 5R chromosome. At least seven co-migrating AFLPs showed the same chromosomal location in both sets of addition lines. A total of 22 AFLPs were assigned to chromosome 1R using wheat-rye substitution lines. Six of them have counterparts in one of the addition lines analyzed, but only four have the same chromosomal location. Six and four of the total AFLPs located using addition (23) and substitution (22) lines segregated in the mapping population DS2 x RXL10, but only six were simultaneously assigned to the same chromosome by both approaches. Although co-migrating AFLPs could be located on different rye chromosomes using addition and substitution lines, we believe that AFLPs can be useful as rye chromosome markers.     

C-banding analysis of gamma-radiation-induced chromosomal interchanges in rye

Gamma-radiation-induced chromosomal interchanges in two inbred lines of rye were analyzed by using the C-banding technique. Neither the doses of radiation nor the time elapsed between radiation and cytological observation influenced the contribution of individual chromosomes to the observed interchanges. Dicentric are more frequent than monocentric interchanges. Neither the individual chromosome contribution to interchanges nor the chromosome combinations fit a random distribution. Chromosomes 6(5R), 4(4R) and 7(1R) of the inbred line Pool and 6(5R), 5(6R) and 4(4R) of line Riodeva, which are carriers of large amounts of heterochromatin, are much more frequently involved in interchanges than expected. Likewise, they are involved in the most frequent combinations observed (6-4, 6-7, 4-5, 4-7 and 6-3 (2R) in line Pool and 6-5, 6-4, 4-5 and 6-7 in line Riodeva). The role of C-heterochromatin in radiation-induced chromosomal interchanges is discussed.     

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.     

Interaction between wheat chromosomes and rye telomeric heterochromatin on meiotic pairing of chromosome pair 1R of rye in wheat-rye derivatives

The effect of telomere heterochromatin on metaphase I association of chromosome pair 1R of rye was analyzed in normal diploid plants of rye (2n=14) and in wheat-rye derivatives with the chromosome constitution (0–7)A(0–7)BRR (2n=20, 21 and 22). The C-banding pattern of 1R was variable between plants. In diploid rye the presence or absence of telomeric heterochromatin in 1R does not influence its meiotic pairing. However, in wheat-rye derivatives the presence of telomeric heterochromatin decreases chiasma frequency in the 1R bivalent. This cannot be attributed to interference of heterochromatin with chiasma terminalization. This effect of heterochromatin is most pronounced in heterozygous condition. In plants heterozygous for telomeric C-bands the reduction of pairing is stronger in the short arm than in the long arm of the 1R bivalent.     

Interaction between rye B-chromosomes and wheat genetic systems controlling homoeologous pairing

The interactive effect on homoeologous pairing of rye B-chromosomes with the absence of both pairing suppressor (3A, 3D, 5B) and promotor (3B, 5A, 5D) chromosomes of common wheat (Triticum aestivum L.) is analyzed by comparison of pairing at Metaphase I of 27-, 27+2B, 28- and 28+2B-chromosome plants. These plants were obtained from crosses between the respective wheat monosomics (2n=41) and rye plants (Secale cereale L.) carrying or not carrying two B-chromosomes (2n=14 or 14+2Bs). —The effect of rye B-chromosomes on pairing depends on the function of the wheat chromosome which is absent in the appropriate hybrids, i.e., rye B-chromosomes have a suppressor effect on pairing when the pairing suppressing wheat chromosomes 3A, 3D or 5B are absent, while they behave as promotors when the pairing promoting chromosomes 3B, 5A or 5D are absent.     

C-banding pattern and meiotic pairing in five rye chromosomes of hexaploid triticale

Summary The meiotic behaviour of rye chromosomes 1R, 2R, 3R, 6R and 7R/4R of hexaploid triticale Cachirulo is analyzed using the C-banding technique. These chromosomes show different C-banding patterns and present different pairing levels at metaphase I. A decreasing effect of large telomeric heterochromatin bands on pairing is deduced from the following two main facts: i) The chromosome 7R/4R shows the highest pairing associated with the smallest amount of heterochromatin, ii) pairing levels of 2 R short arm and 3 R long arm which carry large telomeric bands are less than their respective long and short arms lacking telomeric heterochromatin. Possible desynaptic effects of heterochromatin are discussed although an asynaptic effect cannot be rejected.     

Absence of qualitative genes controlling interspecific pairing in rye B chromosomes

Summary The meiotic behaviour of hybrids between Secale cereale carrying B chromosomes and S. vavilovii has been studied in order to estimate the effects of B chromosomes on hybrid meiotic pairing. The possible effect of Bs on the meiotic pairing of the offspring from backcrosses with S. vavilovii has been studied also. The results obtained clearly indicate that no detectable differences existed in chromosome pairing of hybrids with or without B chromosomes. The hypothetical existence of epistatic genes on cereale genome masking the effect of Bs has been rejected after the results obtained in backcrosses. Therefore, lack of qualitative genes controlling interspecific pairing on rye B chromosomes has been concluded. A quantitative effect of B chromosomes was detected only when they were in alien cytoplasm.     

Chromosome pairing in meiosis of partially fertile wheat/rye hybrids

The normal course of meiosis depends on regular pairing of homologous chromosomes. In intergeneric hybrids, including those of wheat, there is no chromosome pairing because there are no homologs. In F₁ wheat/rye hybrids, pairing is largely prevented by the pairing homoeologous1 (Ph1) gene. In its presence, there are only rare instances of pairing; most chromosomes are univalent, and their orientation at metaphase I initiates different pathways of the meiotic cycle. The meiotic-like pathway includes a combination of the reductional and the equational + reductional steps at AI followed by the second division. The resulting gametes are mostly non-functional. The mitotic-like pathway involves equational division of univalents at AI and the absence of the second division. Any fertility of wheat/rye hybrids depends on the production of unreduced gametes arising from meiotic restitution (mitotic-like division). We examined the meiotic pairing in wheat/rye hybrids created from wheat lines with single rye chromosome substitutions and Ph1 present. This guaranteed F₁ meiosis with one pair of rye homologs. All hybrids formed bivalents, but proportions of meiocytes with bivalents varied. In the meiocytes where bivalents were present, there was a higher tendency for the meiotic-like pathway, while in meiocytes where bivalent pairing failed, the tendency was stronger for the mitotic-like pathway. Among the equationally dividing cells, we observed more than 90 % of meiocytes without bivalents, where rye homologs did not form bivalents, too. The data indicate a potential application of wheat/rye lines in producing genetic stocks of amphidiploids with designated genomic constitutions.     

Alteration of meiotic chromosomal pairing and synaptonemal complexes by cycloheximide

When cells are exposed to cycloheximide during the synaptic period of meiotic prophase, the structure of the synaptonemal complex is markedly altered. The bulk of the lateral component is removed. When lily zygotene microsporocytes are subsequently transferred into a culture medium free from cycloheximide, normal synaptonemal complexes are again seen. Modification of the structure of the synaptonemal complex by treatment with cycloheximide for 4 days has little permanent effect on meiosis except at late zygonema or early pachynema. Treatment at this time produces meiocytes in which no synaptonemal complexes reform. When these cells proceed into diplotene and diakinesis they are devoid of chiasmatic chromosomes. The data suggest that the synaptonemal complex is essential if chiasmata are to be formed, and that a unique period exists when the formation can be interrupted.This work was supported by grants from the National Science Foundation (GB 5173X and GB 6476) and the National Institutes of Health (GM 16882).     

Chromosome structure and pairing preferences in autotetraploid rye (Secale cereale).

The influence of chromosome structure upon pairing behaviour during meiosis was investigated by comparing four autotetraploid genotypes of rye (Secale cereale) containing homologous chromosome sets with different degrees of structural similarity. The series provided a range of genotypes that, at one extreme, contained structurally identical chromosome sets and, at the other extreme, sets that are certainly more heterozygous in the genic sense and probably also more diverse from a purely structural viewpoint. Relative frequencies of pairing configurations at meiotic prophase and metaphase I were compared by electron microscopy of whole-mount surface-spread synaptonemal complex complements and light microscopy of squash preparations. Despite unexpectedly low quadrivalent frequencies over all four genotypes, higher mean bivalent frequencies appeared to be associated with greater homologue diversity. In other words, greater structural divergence between chromosome sets appears to facilitate more efficient discrimination between homologous and identical chromosomes that drives the formation of bivalents. Statistical comparisons were not able to confirm in some cases the significance of the observed pattern of pairing behaviour.     

The relation between pairing preference and chiasma frequency in tetrasomics of rye.

The association pattern of marked tetrasomes of Secale chromosome 1R at meiotic first metaphase was analyzed. Two of the four chromosomes were identical with terminal C-bands at both arms; the other two were also identical but lacked C-bands and were homologous or homeologous with the first two. Four different types of heterozygotes for 1R were studied: (i). autotetraploid hybrids between genetic variants within Secale cereale subsp. cereale, (ii). tetraploid hybrids between subspecies of Secale cereale, (iii). tetraploid hybrids between species of Secale, and (iv). autotetrasomes of S. cereale in a wheat background. Earlier observations that heterozygous associations (banded with unbanded) had consistently higher chiasma frequencies than homozygous associations were extended and confirmed. To analyze this phenomenon more closely, the possible relations between this correlation and several other meiotic phenomena were studied. For this analysis, three genetically different autotetraploid hybrids within S. cereale were selected that differed with respect to the relation between pairing type and chiasma frequency. Special attention was given to different patterns of interference and other meiotic phenomena in the two chromosome arms of chromosome 1R. No relations between such phenomena and the relation between pairing type and chiasma frequency could be established. A hypothesis is formulated assuming that long-distance homologue attraction is concentrated in a limited number of sites and that in different genotypes, different patterns of active sites are present. Moderately weak attraction sites can pair with strong homologous sites under favorable genetic conditions, but two weak sites cannot. Then, heterozygotes have more effective pairing initiation and consequently chiasma formation than homozygotes. Under less favorable conditions, only strong sites are effective, and then, homozygotes pair better, but the chiasma frequency is lower. A model of the forces involved in homologue attraction is presented.     

Effect of a rye B chromosome and its segments on homoeologous pairing in hybrids between common wheat and Aegilops variabilis

Rye B chromosomes, which are supernumerary chromosomes dispensable for the host but increase in number by non-disjunction after meiosis, have been reported to affect meiotic homoeologous pairing in wheat-rye hybrids. The effect of a rye B chromosome (B) and its segments (B-9 and B-10) on homoeologous pairing was studied in hybrids between common wheat (2n=42) and Aegilops variabilis (2n=28), with reference to the Ph1 gene located on wheat chromosome 5B. The B-9 and B-10 chromosomes are derived from reciprocal translocations between a wheat and the B chromosomes, and the former had the B pericentromeric segment and the latter had the B distal segment. Both the B and B-9 chromosomes suppressed homoeologous pairing when chromosome 5B was absent. On the other hand, the B-9 and B-10 chromosomes promoted homoeologous pairing when 5B was present. On pairing suppression, B-9 had a greater effect in one dose than in two doses, and B-9 had a greater effect than B-10 had in one dose. These results suggested that the effect of the B chromosomes on homoeologous pairing was not confined to a specific region and that the intensity of the effect varied depending on the presence or absence of 5B and also on the segment and dose of the B chromosome. The mean chiasma frequency (10.23) in a hybrid (2n=36) possessing 5B and one B-9 was considerably higher than that (2.78) of a hybrid (2n=35) possessing 5B alone, and was comparable with that (14.09) of a hybrid (2n=34) lacking 5B. This fact suggested that the B chromosome or its segment can be used in introducing alien genes into wheat by inducing homoeologous pairing between wheat and alien chromosome.     

Responding to chromosomal breakage during M-phase: insights from a cell-free system

DNA double strand breaks (DSBs) activate ATM and ATR dependent checkpoints that prevent the onset of mitosis. However, how cells react to DSBs occurring when they are already in mitosis is poorly understood. The Xenopus egg extract has been utilized to study cell cycle progression and DNA damage checkpoints. Recently this system has been successfully used to uncover an ATM and ATR dependent checkpoint affecting centrosome driven spindle assembly. These studies have led to the identification of XCEP63 as major target of this pathway. XCEP63 is a coiled-coil rich protein localized at centrosome essential for proper spindle assembly. ATM and ATR directly phosphorylate XCEP63 on serine 560 inducing its delocalization from centrosome, which in turn delays spindle assembly. This pathway might contribute to regulate DNA repair or mitotic cell survival in the presence of chromosome breakage.