THE QUESTION OF POST-REDUCTION IN SPHAGNUM

The 19 spatially distinct chromosomal units at first meiotic metaphase in sporophytically diploid species of Sphagnum have usually been considered to be bivalents, but one investigator (Sorsa, 1956) has interpreted them as chromosomes from dissociated bivalents and meiosis as post-reductional. The present studies on diploid S. squarrosum (Pers.) Crome establish the chromosome number on the basis of the following evidence: there are in addition to m-chromosomes, 19 pairs of chromosomes in early prophase, 19 bivalents at diakinesis, 19 chromosomes in each of the two sets at second metaphase, 19 daughter chromosomes in each of the four sets at late second anaphase, and 19 chromosomes in gametophytic mitoses. The 19 bodies at first meiotic metaphase in diploid species are true bivalents in loose secondary association, which has led to their erroneous interpretation as chromosomes of dissociated bivalents. The gametic chromosome number in sporophytically diploid Sphagnum is therefore, without doubt, n = 19, and this evidence negates the claim for post-reduction in Sphagnum.     

Robertsonian chromosomes and the nuclear architecture of mouse meiotic prophase spermatocytes

Background

The nuclear architecture of meiotic prophase spermatocytes is based on higher-order patterns of spatial associations among chromosomal domains from different bivalents. The meiotic nuclear architecture depends on the chromosome characteristics and consequently is prone to modification by chromosomal rearrangements. In this work, we consider Mus domesticus spermatocytes with diploid chromosome number 2n = 40, all telocentric, and investigate a possible modification of the ancestral nuclear architecture due to the emergence of derived Rb chromosomes, which may be present in the homozygous or heterozygous condition.

Results

In the 2n = 40 spermatocyte nuclei random associations mediated by pericentromeric heterochromatin among the 19 telocentric bivalents ocurr at the nuclear periphery. The observed frequency of associations among them, made distinguishable by specific probes and FISH, seems to be the same for pairs that may or may not form Rb chromosomes. In the homozygote Rb 2n = 24 spermatocytes, associations also mediated by pericentromeric heterochromatin occur mainly between the three telocentric or the eight metacentric bivalents themselves. In heterozygote Rb 2n = 32 spermatocytes all heterochromatin is localized at the nuclear periphery, yet associations are mainly observed among the three telocentric bivalents and between the asynaptic axes of the trivalents.

Conclusions

The Rb chromosomes pose sharp restrictions for interactions in the 2n = 24 and 2n = 32 spermatocytes, as compared to the ample possibilities for interactions between bivalents in the 2n = 40 spermatocytes. Undoubtedly the emergence of Rb chromosomes changes the ancestral nuclear architecture of 2n = 40 spermatocytes since they establish new types of interactions among chromosomal domains, particularly through centromeric and heterochromatic regions at the nuclear periphery among telocentric and at the nuclear center among Rb metacentric ones.     

Cytology of five species of subfamily Papaveroideae from the Western Himalayas

During the present course, population-based meiotic studies were carried out on five species of subfamily Papaveroideae from selected localities of Kashmir and Himachal Pradesh in the Western Himalayas (India). Varied intraspecific chromosome counts were reported for the first time in Argemone mexicana and Meconopsis latifolia, both existing on 2n?=?2x?=?14. The x?=?7, confirmed for the first time from the newly found diploid cytotype, is suggested to be the primary chromosomal basic number for the Meconopsis. Furthermore, meiotic course was noted to be normal in Argemone ochroleuca, it varied from normal to abnormal in the populations of A. mexicana and Papaver dubium whereas it was invariably found to be abnormal in all the populations of Meconopsis aculeata and M. latifolia. These anomalous taxa were marked with meiotic abnormalities in the form of cytomixis, chromosomal stickiness, unoriented bivalents, formation of laggards and bridges resulting in abnormal microsporogenesis, and production of heterogeneous-sized fertile pollen grains along with reduced pollen fertility.     

Cytomorphological diversity in some selected members of Poaceae from Parvati Valley in Kullu district of Himachal Pradesh,India

The present work includes detailed male meiotic studies on 46 species of grasses falling into 59 accessions from different localities of Parvati Valley in Kullu district of Himachal Pradesh in the altitudinal range of 1,100 to 2,750 m. All the species have been studied cytologically for the first time from the study area. The meiotic chromosome count of n = 14 for Calamagrostis emodensis is the first ever chromosome report. Three species, namely Agrostis alba (n = 21), Avena byzantina (n = 21) and Bromus inermis (n = 14) have been studied cytologically for the first time from India. New intraspecific diploid/polyploid cytotypes have been reported for Arthraxon serrulatus (2n = 4x = 32), Iseilema laxum (2n = 12x = 60), Digitaria albudens (2n = 8x = 72), Festuca kashmiriana (2n = 2x = 14) and Stipa orientalis (2n = 2x = 20). The existence of variable number of B-chromosomes (2n = 60 + 0-5B) has been reported for the first time in the 12x cytotype of Iseilema laxum. Secondary associations of chromosomes in the tetraploid cytotype of Cymbopogon martini (n = 20) indicated its secondary polyploid nature. As many as 18 species showed various meiotic anomalies such as the phenomenon of cytomixis involving inter PMC migration of chromatin material, chromatin stickiness, interbivalent connections, abnormal spindle activity, presence of bridges and laggards during anaphases and telophases and abnormal sporads. These meiotic abnormalities consequently yielded sterile and heterogeneous-sized fertile pollen grains. The polyploidy and aneuploidy have played an active role in the evolution of grasses.     

Cytology of the genus Artemisia (Anthemidae,Asteraceae) in the Western Himalayas

The present study revealed the varied frequency of natural chromosomal abnormalities in 13 populations pertaining to 9 species of the genus Artemisia L. from different localities of Himachal Pradesh (Western Himalaya). Intraspecific chromosome variability has been reported for the first time on worldwide basis in Artemisia vestita (2n = 2x = 36) and from India in A. macrocephala (2n = 2x = 18) and A. scoparia (2n = 2x = 36). Besides, B-chromosomes have been reported here for the first time in A. nilagirica and A. roxburghiana. Most of the populations show anomalous meiotic behaviour resulting in cytomixis, chromosomal stickiness, unoriented bivalents, formation of laggards and bridges which leads to abnormal microsporogenesis, and production of heterogeneous-sized fertile pollen grains along with reduced pollen fertility.     

Meiotic behavior and pollen viability of tetraploid Arachis glabrata and A. nitida species (Section Rhizomatosae, Leguminosae): implications concerning their polyploid nature and seed set production

The meiotic behavior and pollen viability of the tetraploids (2n?=?4x?=?40) Arachis glabrata and A. nitida were analyzed aiming to provide insights into the nature of these polyploids and into the mechanism that determines the low seed production of these species. Meiotic analysis revealed 21 different chromosome configurations at diakinesis-metaphase I in A. glabrata (from 20 II to 4 II?+?8 IV) and 7 in A. nitida (from 20 II to 2 I?+?12 II?+?2 III?+?2 IV). The multivalent associations (up to 8 IV) observed in some A. glabrata metaphases suggest that this species may have an autopolyploid origin. However, the fact that the mean number of bivalents varied among accessions indicates different degrees of diploidization among them. In contrast, the multivalents (up to 2 III?+?2 IV) observed in very low frequency in A. nitida indicate that this species may be either a largely diploidized autopolyploid or a segmental allopolyploid. A great diversity, although in low frequency, of meiotic abnormalities (abnormal chromosome orientation and segregation, chromosome bridges, irregular spindles, micronuclei, aneuploid nuclei, restitution nuclei, microcytes, monads, dyads, triads, and hexads) was detected in both species. The meiotic indexes were over 95%, and pollen viabilities ranged from 83.20 to 95.99%. These results suggest that chromosome behavior during meiosis would not severely affect pollen viability. Thus, the irregular chromosome behavior caused by the autopolyploid or segmental allopolyploid nature of these species may not be related to their low seed set production.     

Male meiosis and behaviour of sex chromosomes in different populations of Rumex acetosa L. from the Western Himalayas, India

Detailed meiotic analysis in 28 North West Himalayan populations of dioecious plant Rumex acetosa L. was carried out. The species is generally discussed as an important plant having sex chromosomes. Male meiosis in all the studied populations clearly showed the formation of six bivalents and one trivalent during diakinesis and metaphase-I. The sex chromosomes in male plants exhibit a chain of trivalent (Y₁–X–Y₂). In addition, among the presently investigated populations ring-shaped trivalents were also observed for the first time in the species. Varied frequency of abnormal segregation of sex trivalent was also observed leading to XY:Y segregation instead of normal X:Y₁Y₂ segregation. A majority of the populations exhibit normal meiosis. Plants of six populations show meiotic abnormalities like cytomixis, laggards, bridges, chromatin stickiness, etc., leading to reduced pollen fertility. Translocation between an autosome and sex chromosomes was also observed in some of the populations. 0–1B chromosomes were noticed in one population. This is the first ever meiotic analysis of the species from India.     

Persistent occurrence of meiotic abnormalities in a new hexaploid cytotype of <Emphasis Type="Italic">Thalictrum foetidum</Emphasis> from Indian cold deserts

Thalictrum foetidum L. (Ranunculaceae), a morphologically variable and widely distributed species of temperate and alpine Himalayas is worked out cytologically for the first time from India. Earlier studies from outside India were restricted to chromosome counts and karyotypic analysis. We studied the male meiosis, microsporogenesis and pollen viability in the wild accessions from the cold deserts of Lahaul-Spiti, Kinnaur and Pangi Valley of Himachal Pradesh. Present cytomorphological surveys in the species record the existence of two distinct morphotypes involving plant size; colour and size of leaf/leaflet; dentation of leaflet lobes; and degree of leaf pubescence. All the accessions in the two morphovariants share the same meiotic chromosome number (n = 21) and adds a new intraspecific hexaploid cytotype. The accessions show the phenomenon of cytomixis involving transfer of chromatin material among proximate pollen mother cells (PMCs) and associated meiotic abnormalities like, out of plate bivalents, interchromosomal connections, and laggards, bridges and micronuclei at anaphases/telophases. Microsporogenesis results into abnormal sporads (tetrads with micronuclei, dyads, triads and polyads). The products of such sporads resulted into some pollen sterility and pollen grains of heterogeneous sizes. The persistent occurrence of phenomenon of cytomixis and associated meiotic abnormalities and consequently pollen sterility and pollen grains of heterogeneous sizes in the hexaploid cytotype of T. foetidum seems to be under some genetic factors associated with the genome.     

Meiosis and pollen mitosis in diploid and triploid Colocasia antiquorum Schott.

The relationship between diploid and triploid forms of Colocasia antiquorum Schott. was assessed through comparative meiotic and pollen mitotic studies. Owing to poor spreading of the chromosomes of both materials, karyological observations on pachytene nuclei were limited to a few chromosomes. Among the two nucleolar chromosomes and a metacentric, telochromomere-bearing chromosome of the diploid, the latter and one of the nucleolar chromosomes characterized by a heteropycnotic short arm were identified in both bivalent and trivalent associations in the triploid. The homologues in these cases were homomorphic and intimately paired. Two types of heteromorphic bivalents exhibiting partial pairing of homomorphic segments were also recorded in the triploid. Among the 14 bivalents of the diploid at diakinesis, two were nucleolus-associated. In the triploid, chromosomal associations at diakinesis included trivalents (2 to 9), bivalents and univalents, and the chiasma frequency per paired chromosome was lower than in the diploids. In 21.6 percent of the PMCs at this stage intragenomic pairing of one or two chromosomes was observed. Post-diakinesis stages in the diploid were regular while in the triploid they were marked by various irregularities in a majority of the cells. However, fertility (stainability), size and divisional frequency of pollen in both materials were remarkably similar. Chromosome numbers in pollen nuclei in the triploid ranged from 8 to 25. Based on these data an autopolyploid origin for the triploid Colocasia and a lower base number than the gametic chromosome number for this genus are advanced.     

First detection of cytomixis and its consequences in <Emphasis Type="Italic">Thalictrum cultratum</Emphasis> Wall. (Ranunculaceae)

In current paper, we studied the meiotic chromosome number and details of cytomixis recorded for the first time in Thalictrum cultratum Wall. from the high altitude areas of northwest Himalaya (India). Cytomixis, the phenomenon of inter PMC (pollen mother cell) migration of chromatin material has been reported in many angiosperms and other groups of plants, but there is no published report in Thalictrum cultratum. All the presently studied accessions of the species existed at diploid level (x = 7). The present chromosome count of n = 14 has been reported for the first time from the study area. The cytological stability of any plant is an important consideration in view of its existence in nature. Meiotic disturbances can impose threat to the reproductive potential of plant. Current study reveals the occurrence of inter PMC migration of chromatin material. In present investigation, the phenomenon of cytomixis can be observed between 2 to 9 PMCs. Chromatin migration occurs through narrow and broad cytoplasmic channels or occasionally fused together during metaphase-I and anaphase-I. Cytomixis resulted in the formation of hypo- (n = 2, 2 + 1 small fragment, n = 3 + 3 small fragments, 4, 5 + extra mass of chromatin, 13) and hyperploid (n = 7 + 2 small fragments, 8 + 2 small fragments, 14, 14 + 2 small fragments, 15) PMCs, variable sized pollen grains and pollen sterility (20%). The current study reveals that cytomixis is responsible for formation of PMCs with variable chromosomes (hypoand hyperploid) and pollen grains of heterogeneous sizes and pollen sterility in T. cultratum. This is the first report of cytomixis in the species.     

The karyotype of Nodipecten nodosus (Bivalvia: Pectinidae)

Earlier karyotypical work on Nodipecten nodosus embryos indicated that this species has a diploid number of 38, with six pairs respectively of metacentric and submetacentric chromosomes and seven pairs of subtelocentric chromosomes, although there were some difficulties in obtaining complete metaphases. The present work provides additional results on specific regions of the chromosomes in N. nodosus and, by meiotic studies, confirms the chromosome number with more reliability. Active nucleolar organizer regions (NOR), detected in mitotic metaphases from embryos, can be characterized in N. nodosus by a high level of heteromorphism of NOR-sites, indicating that these regions are not appropriate as chromosomal markers in this species. The procedure for detecting constitutive heterochromatin of chromosomes allowed us to observe most of the heterochromatic blocks at a pericentromeric position and some at telomeric and interstitial positions. The analysis of meiotic chromosomes from gonad tissue revealed the presence of 19 bivalents during metaphase I, all homomorphic and isopicnotic, confirming the previously described diploid chromosomal number of 38 for N. nodosus. From these results, some evolutionary aspects of the Pectinidae are briefly discussed.     

The mechanism of secondary nondisjunction in Drosophila melanogaster females

Bridges (1916) observed that X chromosome nondisjunction was much more frequent in XXY females than it was in genetically normal XX females. In addition, virtually all cases of X nondisjunction in XXY females were due to XX <--> Y segregational events in oocytes in which the two X chromosomes had failed to undergo crossing over. He referred to these XX <--> Y segregation events as "secondary nondisjunction." Cooper (1948) proposed that secondary nondisjunction results from the formation of an X-Y-X trivalent, such that the Y chromosome directs the segregation of two achiasmate X chromosomes to opposite poles on the first meiotic spindle. Using in situ hybridization to X and YL chromosomal satellite sequences, we demonstrate that XX <--> Y segregations are indeed presaged by physical associations of the X and Y chromosomal heterochromatin. The physical colocalization of the three sex chromosomes is observed in virtually all oocytes in early prophase and maintained at high frequency until midprophase in all genotypes examined. Although these XXY associations are usually dissolved by late prophase in oocytes that undergo X chromosomal crossing over, they are maintained throughout prophase in oocytes with nonexchange X chromosomes. The persistence of such XXY associations in the absence of exchange presumably facilitates the segregation of the two X chromosomes and the Y chromosome to opposite poles on the developing meiotic spindle. Moreover, the observation that XXY pairings are dissolved at the end of pachytene in oocytes that do undergo X chromosomal crossing over demonstrates that exchanges can alter heterochromatic (and thus presumably centromeric) associations during meiotic prophase.     

Evidence for Emergence of Sex-Determining Gene(s) in a Centromeric Region in Vasconcellea parviflora

Sex chromosomes have been studied in many plant and animal species. However, few species are suitable as models to study the evolutionary histories of sex chromosomes. We previously demonstrated that papaya (Carica papaya) (2n = 2x = 18), a fruit tree in the family Caricaceae, contains recently emerged but cytologically heteromorphic X/Y chromosomes. We have been intrigued by the possible presence and evolution of sex chromosomes in other dioecious Caricaceae species. We selected a set of 22 bacterial artificial chromosome (BAC) clones that are distributed along the papaya X/Y chromosomes. These BACs were mapped to the meiotic pachytene chromosomes of Vasconcellea parviflora (2n = 2x = 18), a species that diverged from papaya ∼27 million years ago. We demonstrate that V. parviflora contains a pair of heteromorphic X/Y chromosomes that are homologous to the papaya X/Y chromosomes. The comparative mapping results revealed that the male-specific regions of the Y chromosomes (MSYs) probably initiated near the centromere of the Y chromosomes in both species. The two MSYs, however, shared only a small chromosomal domain near the centromere in otherwise rearranged chromosomes. The V. parviflora MSY expanded toward the short arm of the chromosome, whereas the papaya MSY expanded in the opposite direction. Most BACs mapped to papaya MSY were not located in V. parviflora MSY, revealing different DNA compositions in the two MSYs. These results suggest that mutation of gene(s) in the centromeric region may have triggered sex chromosome evolution in these plant species.     

Meiosis in haploid barley — An interpretation of non-homologous chromosome associations

Detailed meiotic studies were conducted on ten haploid plants representing six different genotypes of barley (Hordeum vulgare, 2n=14). At pachytene stages the non-homologous chromosomes were observed to pair as intimately as homologous chromosomes in many cells. Foldback pairing, involving single chromosomes, and multivalent associations were common. At diplotene, up to 4 chiasmatalike structures were observed in paired chromosomes but it is not likely that they resulted from crossing over. At diakinesis the bivalent frequency mean was from 1 to 1.3 per cell whereas by metaphase I the paired associations were rare with a single rod bivalent being observed in 3 to 5% of the cells. The frequencies of various types of secondary associations at metaphase were also recorded. — The origin and significance of bivalents and secondary associations in haploids is reviewed and discussed. Caution is urged in the interpretation that low levels of chromosome pairing in haploids is evidence of homology. It is concluded that very little chromosome duplication is likely to be found within the haploid set of barley chromosomes and that the basic chromosome number is seven.     

C-banding and non-homologous associations in Gryllus argentinus

A comparative study of C-banded mitotic and meiotic chromosomes of Gryllus argentinus (Gryllidae) is reported. Improved cytological procedures were followed to establish karyotypes and to detect C-segments. Twenty-eight autosomes plus a sexual system XX, XO were found. Terminal C-heterochromatin in both arms and paracentric segments were detected in most of the chromosomes of the complement. Microdensitometric tracings confirmed the distribution of C-banded segments. Manifold connections through condensed terminal chromomeres were observed at pachytene occurring between two or more bivalents during meiosis. These heterologous associations involved the whole karyotype. End-to-end pachytene associations reacted intensely to the C-procedure. C-segments detected at diakinesis allowed the measurement of the centromere indices of bivalents which resulted in a more precise identification of given chromosome pairs. The relationship of the presence of terminal heterochromatin in mitotic chromosomes, the end-to-end associations through C-segments, the attachment of pachytene filaments to the nuclear membrane and their molecular implications is discussed.     

Cytogenetic studies on <Emphasis Type="Italic">Metasequoia glyptostroboides</Emphasis>, a living fossil species

The chromosome morphology and meiotic pairing behavior in the pollen mother cells (PMCs) of Metasequoia glyptostroboides were investigated. The results showed that: (1) The chromosome number of the PMCs was 2n=22. (2) The PMCs developed in the successive manner, and the nucleoids in the dynamic development were similar to those of the other gymnosperms. (3) At prophase, most of the chromosomes were unable to be identified distinctively because the chromosomes were long and tangled together. The chromosome segments were paired non-synchronously. At pachytene, the interstitial or terminal regions of some bivalents did not form synapsis and the paired chromosomes showed difference in sizes, indicating that there were structure differences between the homologous chromosomes. (4) At diakinesis, the ring bivalents showed complicated configurations due to the differences in location and number of chiasmata. In addition, there were cross-linked bivalents. (5) At metaphase I, the chromosome configuration of each cell was 8.2II ⁰ + 1.1II + 1.3II ⁺ + 0.8I. Most of the chromosomes were ring bivalents, but some were cross-linked bivalents, rod bivalents, or univalents. (6) 15\% PMCs at anaphase I and 22\% PMCs at anaphase II presented chromosome bridges, chromosome fragments, micronuclei, and lagging chromosomes. Twenty seven percent microspores finally moved into one to three micronuclei. Twenty five percent pollens were abortive. The results indicated that the observed individual of M. glyptostroboideswas probably a parpcentric inversion heterozygote, and there were structural and behavioral differences between the homologous chromosomes. The chromosomal aberration of M. glyptostroboidesmay play an important role in the evolution of this relict species, which is known as a living fossil. Further evidence is needed to test whether the differences between homologous chromosomes were due to hybridization.     

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.     

Cytogenetics of Manihot esculenta Crantz (cassava) and eight related species

Thirty-nine cultivars of cassava and eight related wild species of Manihot were analyzed in this work for number, morphology and size of chromosomes, prophase condensation pattern and the structure of the interphase nucleus. In four accessions, the chromosome size was measured and in some others, the number of secondary constrictions, meiotic behavior, C-band pattern, CMA/DAPI bands, nucleoli number and the location of 5S and 18S-5.8S-28S rDNA sites were also observed. All investigated accessions showed a similar karyotype with 2n = 36, small metacentric to submetacentric chromosomes. Two pairs of terminal secondary constrictions were observed in the chromosome complement of each accession except Manihot sp. 1, which presented two proximal secondary constrictions. The prophase chromosome condensation pattern was proximal and the interphase nuclei structure was areticulate to semi-reticulate. The meiosis, investigated in seven cultivars and four wild species, was regular, displaying 18 bivalents. C-banding revealed heterochromatin in 9 or 10 chromosomes. The analysis with fluorochromes frequently showed four chromosome pairs with a single CMA+ terminal or subterminal band and a few other chromosomes with DAPI+ unstable bands. Six 45S rDNA sites were revealed by FISH, which seemed to colocalize with six CMA+ bands. Only one chromosome pair presented a 5S rDNA site. The maximum nucleoli number observed per nucleus was also six. These data suggest that all Manihot species present a very similar chromosome complement.     

Translocations of Chromosome End-Segments and Facultative Heterochromatin Promote Meiotic Ring Formation in Evening Primroses

Due to reciprocal chromosomal translocations, many species of Oenothera (evening primrose) form permanent multichromosomal meiotic rings. However, regular bivalent pairing is also observed. Chiasmata are restricted to chromosomal ends, which makes homologous recombination virtually undetectable. Genetic diversity is achieved by changing linkage relations of chromosomes in rings and bivalents via hybridization and reciprocal translocations. Although the structural prerequisite for this system is enigmatic, whole-arm translocations are widely assumed to be the mechanistic driving force. We demonstrate that this prerequisite is genome compartmentation into two epigenetically defined chromatin fractions. The first one facultatively condenses in cycling cells into chromocenters negative both for histone H3 dimethylated at lysine 4 and for C-banding, and forms huge condensed middle chromosome regions on prophase chromosomes. Remarkably, it decondenses in differentiating cells. The second fraction is euchromatin confined to distal chromosome segments, positive for histone H3 lysine 4 dimethylation and for histone H3 lysine 27 trimethylation. The end-segments are deprived of canonical telomeres but capped with constitutive heterochromatin. This genomic organization promotes translocation breakpoints between the two chromatin fractions, thus facilitating exchanges of end-segments. We challenge the whole-arm translocation hypothesis by demonstrating why reciprocal translocations of chromosomal end-segments should strongly promote meiotic rings and evolution toward permanent translocation heterozygosity. Reshuffled end-segments, each possessing a major crossover hot spot, can furthermore explain meiotic compatibility between genomes with different translocation histories.