Meiotic differences between three triatomine species (Hemiptera,Reduviidae)

We have found the following differences in the male meiosis among three triatomine species: (1) The three largest autosomal bivalents ofTriatoma infestans are heterochromatic.Rhodnius prolixus has two autosomal bivalents with heterochromatic blocks.Triatoma rubrovaria does not show any heteropycnotic autosomes. (2) Sex chromosomes inT. infestans form a chromocenter. At early prophase terminal associations are seen between sex chromosomes inT. rubrovaria, and they maintain a close association until diakinesis. An intimate association between the X and Y chromosomes is observed during early prophase inR. prolixus, but a distant association is maintained by the sex chromosomes at diffuse and diplotene stages in this species. (3) Polyploid nuclei of the nutritive cells are quite distinct. Numerous chromocenters of different shapes and sized are seen in those ofT. infestans. InT. rubrovaria one chromocenter having two positively heteropycnotic elements is observed surrounded by homogeneous chromatin. Only one compact chromocenter is found amongst unevenly distributed chromatin, inR. prolixus.     

Comparison of the chromosomes of Triticum timopheevi with related wheats using the techniques of C-banding and in situ hybridization

Summary The chromosomes of the tetraploid wheats Triticum timopheevi (Genome AAGG) and T. araraticum (Genome AAGG) were C-banded at mitosis. The identity of the banded and unbanded chromosomes was then established by firstly making comparisons with the hexaploid species T. zhukovskyi which has the genome formula AAAAGG. Secondly, the meiotic pairing in F₁ hybrids between T. timopheevi and diploid wheats was examined by means of C-banding. The results showed that the banded chromosomes belonged to the G genome, while the unbanded chromosomes belonged to the A genome. Only one of the two pairs of satellited chromosomes had strong heterochromatic bands. The relationship between the genomes of T. timopheevi and T. dicoccum (Genome AABB) was then assessed at meiosis in hybrids between these species, using the techniques of C-banding and in situ hybridisation of a cloned ribosomal RNA gene probe. It was concluded that there were differences both in the amount and distribution of heterochromatin and also translocation differences between the species.     

Genomic compatibility between two phyllotine rodent species evaluated through their hybrids

In order to investigate the genomic compatibility between allopatric rodent species, Phyllotis darwini and Phyllotis magister, we have studied several cytogenetic and reproductive features of their laboratory hybrids. Of thirty-one pairings between species, only five were successful, producing eleven newborns. Like parents, hybrids had 38 metacentric chromosomes, except for the subtelocentric Y chromosome inherited from P. magister. There was almost total C and G band correspondence between homeologous autosomes. However, parental sex chromosomes had different morphology, C and G bands. Ag-NOR bands appeared as small telomeric Ag+ regions, distributed in four chromosomal pairs of darwini, three of magister and four homeologous chromosomes of the hybrids. The three forms had similar indexes of NOR activity per cell, in spite of the variability in NOR expression which was always detected. Usually, only one member of parental homologous chromosomes showed AgNOR+; nevertheless, both homeologous chromosomes were active in many hybrid cells. The frequencies of cells that expressed their ribosomal genes in the two homologous or homeologous NOR chromosomes were similar in parental and hybrid cells. These results strongly suggest that ribosomal genes of both parental genomes would function codominantly in the hybrids. The gonad histological and morphometric analyses showed that hybrids conformed to Haldane's rule, since females were fertile and males were infertile. Our results indicate that P. darwini and P. magister genomes can function in relative harmony and compatibility when they are placed together in their laboratory generated hybrids, suggesting that these species have few genetic differences, probably because they have recently diverged.     

The cytology of Brachycome line ariloba

Hybrids between B. campylocarpa (2n=8) and B. lineariloba Race A (2n=4) have been made and studied in both meiosis and mitosis. In B. campylocarpa A × B. lineariloba A₃, meiotic pairing is limited, and suggests that there are segmental homologies between the two lineariloba chromosomes and three of the four campylocarpa chromosomes. The chromosomal relationship between the two species is not close. In hybrids there is a marked difference or asynchrony in the condensation patterns of the chromosomes from the two parental sources. Condensation behaviour is not determined by the genome and must be under local control within each chromosome.     

Meiotic pairing of the amphiploid Hordeum chilense X Triticum turgidum conv. durum studied by means of Giemsa C-banding technique

Summary The meiotic behaviour of the amphiploid Hordeum chilense X Triticum turgidum conv. durum using a C-banding staining method is studied. Nine pairs of chromosomes at metaphase-1 (4A, 7A and the seven of the B genome) were identified and the remaining wheat chromosomes (1A, 2A, 3A, 5A and 6A) and seven of the chilense (1 to 7 H ^ch chromosomes) were assigned to its particular genome. A similar mean number of univalents from parental genomes (wheat and wild barley) were found. No meiotic pairing between chilense and turgidum chromosomes was detected. Differences in the meiotic behaviour per chromosome and amongst genomes are explained on the basis of cytomorphological and heterochromatin characteristics.     

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.     

A comparative study of the recombination pattern in three species of Platyrrhini monkeys (primates)

Homologous chromosomes exchange genetic information through recombination during meiotic synapsis, a process that increases genetic diversity and is fundamental to sexual reproduction. Meiotic studies in mammalian species are scarce and mainly focused on human and mouse. Here, the meiotic recombination events were determined in three species of Platyrrhini monkeys (Cebus libidinosus, Cebus nigritus and Alouatta caraya) by analysing the distribution of MLH1 foci at the stage of pachytene. Moreover, the combination of immunofluorescence and fluorescent in situ hybridisation has enabled us to construct recombination maps of primate chromosomes that are homologous to human chromosomes 13 and 21. Our results show that (a) the overall number of MLH1 foci varies among all three species, (b) the presence of heterochromatin blocks does not have a major influence on the distribution of MLH1 foci and (c) the distribution of crossovers in the homologous chromosomes to human chromosomes 13 and 21 are conserved between species of the same genus (C. libidinosus and C. nigritus) but are significantly different between Cebus and Alouatta. This heterogeneity in recombination behaviour among Ceboidea species may reflect differences in genetic diversity and genome composition.     

DYNAMICS OF A NOVEL CHROMOSOMAL POLYMORPHISM WITHIN A HYBRID ZONE BETWEEN TWO CHROMOSOME RACES OF THE SCELOPORUS GRAMMICUS COMPLEX (SAURIA,PHRYNOSOMATIDAE)

Several chromosome races of the mesquite lizard, Sceloporus grammicus complex, hybridize at localities in central Mexico. In most cases, the hybridizing populations are delineated by centric fissions at one or more of the macrochromosomes. One notable exception is the Tulancingo hybrid zone between the F5 and FM2 cytotypes. In addition to fission and/or inversion differences at chromosomes 1, 3, 4, and 6, these races differ by a complex rearrangement of chromosome 2, which carries the nucleolus-organizer region in this species. The meiotic consequences of heterozygosity at this chromosome were examined in males to assess the potential for this chromosome to contribute to the dynamics of the hybrid zone. Chromosomal analysis revealed several putative F₁ hybrids and confirmed the production of nonparental chromosomal morphologies through recombination. Pachytene analysis revealed meiotic pairing difficulties associated with chromosome 2 in males heterozygous for the parental chromosomal morphologies. Significant aneuploidy is expected because of random disjunction of the chromosome-2 elements. As a result, these males likely suffer reduced fertiliity and fitness. In contrast, males heterozygous for recombinant chromosomal morphologies displayed low levels of meiotic irregularities and presumably exhibit higher fertility than individuals heterozygous for parental morphologies. It is hypothesized that the recombinant phenotypes facilitate gene flow between the F5 and FM2 cytotypes.     

Reduced meiotic fitness in hybrids with heterozygosity for heterochromatin in the speciating<Emphasis Type="Italic">Mus terricolor</Emphasis> complex

Mus terricolor I, II and III are the three chromosomal species which differ in stable autosomal short-arm heterochromatin variations established in homozygous condition. Analysis of meiosis in the laboratory-generated F₁ male hybrids from crosses (both ways) betweenM. terricolor I and II and betweenM. terricolor I and III shows high frequencies of pairing abnormalities at pachytene. The backcross (N3 generation) male hybrids betweenM. terricolor I and II have meiotic abnormalities as in the F₁male hybrids, though to a lesser extent. They show difference in pairing abnormalities in the different karyotypic forms; the backcross hybrids heterozygous for the heterochromatic short arms have more anomalies compared to the homokaryotypic hybrids. This suggests a negative influence of the heterochromatin heterozygosity in meiotic pairing. The results indicate a role for heterochromatin variations in the development of a reproductive barrier in the speciatingM. terricolor complex.     

Karyotype differentiation of three anopheline taxa in the balabacensis complex of Southeast Asia (Diptera: Culicidae)

Cytological studies of the mitotic and meiotic karyotypes and the polytene salivary chromosomes of Anopheles dirus, An. takasagoensis and An. balabacensis (Perlis form), have revealed significant differences in the sex chromosomes. These differences are largely due to the position of the centromere and different amounts of constitutive heterochromatin and euchromatin. The chromosomal data suggest that An. dirus and An. takasagoensis are more closely related to each other than either is to An. balabacensis (Perlis form). The karyological differences are very useful in differentiating these taxa, particularly the Perlis form, and lend support for their species status.     

Production and cytogenetics of the intergeneric hybrids Brassica juncea×Orychophragmus violaceus and B. carinata×O. violaceus

 Intergeneric hybrids between Brassica juncea (2n=36), B. carinata (2n=34) and Orychophragmus violaceus (2n=24) were produced when B. juncea and B. carinata cultivars were used as female parents. The hybrids between B. juncea and O. violaceus had an intermediate morphology except for petal colour and were partially fertile. The hybrids between B. carinata and O. violaceus had a matroclinous morphology and were nearly fertile. Cytological analysis of the hybrids and their progenies gave the following results. (1) In the hybrids between B. juncea and O. violaceus, the somatic tissues of the roots, leaves and styles were mixoploid (2n=12–42), and cells with 24, 30 or 36 chromosomes were the most frequent. Based on the recorded numbers and behaviour of the mitotic and meiotic chromosomes, complete and partial separation of the parental genomes was proposed to have occurred during mitosis. This resulted in the occurrence of cells with possibly complete and incomplete complements of the parental species and cells with parental complements and some additional chromosomes from the other parent. (2)  Pollen mother cells (PMCs) possibly with both parental chromosome complements, only B. juncea chromosomes or a complete B. juncea complement with additional O. violaceus chromosomes were more competitive in entering meiosis. The majority of fertile gametes were deduced to have been produced by PMCs with a B. juncea complement with or without additional O. violaceus chromosomes. (3) The progeny plants from selfed hybrids between B. juncea and O. violaceus were morphologically either of a B. juncea, hybrid or variable type. Cytologically they were grouped into six types according to the frequencies of cells with various chromosome numbers. All of the plants except 2 which constituted two types, were mixoploids, composed of cells with various chromosome numbers, mainly in a certain serial range. (4) The hybrid plants between B. carinata and O. violaceus were mixoploids with chromosome numbers in the range of 12–34, and cells with 2n=34 were the most frequent. The main categories of PMCs with 17 bivalents at metaphase I and 17 : 17 segregations at anaphase I contributed to the high fertility of the hybrids and the fact that their progeny after selfing were mainly plants with 2n=34. Somatic and meiotic separation of the parental genomes was proposed to have occurred in the hybrids between B. carinata and O. violaceus. (5) Mitotic and meiotic elimination of what could be O. violaceus chromosomes might also have contributed to the observed mitotic and meiotic cell types in the two kinds of hybrids studied. Finally, the possible mechanisms behind these cytological observations and their potential in the production of Brassica aneuploids were discussed. Received: 4 February 1997/Accepted: 29 July 1997     

Genetic differentiation in theAedes atropalpus complex. II. Chromosomal divergence betweenAe. atropalpus andAe. epactius

Analysis of meiotic chromosomes from hybrids betweenAedes atropalpus andAe. epactius has revealed that the two species are fixed for alternate arrangements of four inversions: a paracentric inversion of chromosome 1, two paracentric inversions of chromosome 2, and a pericentric inversion of chromosome 3. This chromosomal heterozygosity in the interspecific hybrids has resulted in extensive meiolic chromosomal asynapsis. Dicentric bridges, acentric fragments, and chromosomal breakage were also associated with the heterozygous inversions. This disruption of meiosis was sufficient to account for the partial sterility observed in interspecific hybrids. No chromosomal polymorphisms, aberrations, or reduction in fertility was observed in parental strains of intraspecific hybrids of the two species.     

CYTOLOGICAL ANALYSIS OF ANTHURIUM ANDRAEANUM (ARACEAE), ITS RELATED TAXA AND THEIR HYBRIDS

Karyotypes and meiotic configurations of Anthurium andraeanum and closely related taxa were analyzed. The karyotypes of A. andraeanum, A. caperatum, A. formosum, A. kamemotoanum, A. lindenianum, A. roseospadix, A. cf. sanctifidense, A. subsignatum, A. garagaranum, and an unidentified Anthurium sp. commonly consisted of four large metacentric or submetacentric chromosomes, two fairly large acrocentric chromosomes, two satellite chromosomes, and 22 smaller chromosomes. Variation in the karyotypes of A. nymphaeifolium and A. ochranthum suggested chromosomal rearrangement in the genus. All taxa showed 15 pairs of chromosomes at prometaphase I of meiosis in pollen mother cells. Four large chromosomes appeared as ring bivalents, and the rest of the chromosomes appeared as either ring or rod bivalents. Regular bivalent formation at prometaphase I of meiosis in pollen mother cells of species hybrids suggested close genomic relationships among parental taxa. On the other hand, reduction of pollen fertility estimated by pollen stainability in those hybrids suggested genetic divergence of species.     

Genetic dissection of heterochromatin in Drosophila: The role of basal X heterochromatin in meiotic sex chromosome behaviour

We have examined the female meiotic behaviour of three X chromosomes which have large deletions of the basal heterochromatin in Drosophila melanogaster. We find that most of this heterochromatin can be removed without substantially altering pairing and segregation of the two Xs. To compare the role of heterochromatin in male meiosis we have constructed individuals which carry two extra identical heterochromatic mini X chromosomes. These minis behave as univalents even though their heterochromatin is known to contain satellite DNA. We conclude therefore that this satellite DNA is not sufficient to allow effectively normal meiotic behaviour. In all other respects our results in the male extend and confirm Cooper's postulate that there exist specific pairing sites in the X heterochromatin. Thus we find no support in either female or male meiosis for the concept that satellite DNA is involved in meiotic chromosome pairing of either a chiasmate or an achiasmate kind.     

On the origin of the cyanogenic polymorphism in Trifolium repens L.

Trifolium repens L. and Trifolium nigrescens Viv. are two of the approximately six cyanogenic species known in the genus Trifolium. The two species are closely related: T. nigrescens is considered to be one of the diploid ancestors of the amphidiploid T. repens . We studied morphology, meiosis and the cyanogenic system in T. repens (amphidiploid), T. nigrescens (diploid) and their reciprocal hybrids. A comparison of the enzyme linamarase in the species and hybrids shows that there is a general resemblance between their linamarases. Immunological studies indicate that the linamarases must have a somewhat different three-dimensional structure. These facts are consistent with the view that T. nigrescens (or an ancestral form of this species) has donated the Li gene of T. repens . The other putative parent, T. occidentale has probably not donated an active Li gene. The hypothesis of the origin of the Li gene does not explain its polymorphism in European populations of T. repens , as T. nigrescens is monomorphic for cyanogenesis and amphidiploids do not segregate for genes which are homozygous dominant in one of the parents. Segregation for Li could be caused by a gene mutation or a small exchange between homeologous chromosomes. The latter event is more probable. A nigrescens-repens exchange would give rise to a chromosomal region with reduced homology to both parental chromosomes. The genes in the region of exchange will be tightly linked due to diminished cross-over frequency. It has been known for years that Li has effects on the vegetative and reproductive characters in T. repens and we have recently shown that these effects must be the result of genes linked to Li . As the associated characters influence the fitness of the cyanotypes, not only the origin but also the maintenance of the cyanogenic polymorphism is closely related to the evolutionary history of T. repens .     

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.     

Structural and numerical chromosomal polymorphism in natural populations ofAlopecurus (Poaceae)

Considerable structural and numerical chromosomal variation has been found in natural populations ofAlopecurus. Interchange heterozygotes, identified by multivalent formation during meiosis, have been recovered in four out of six species studied and are reported for the first time in the diploidsA. bulbosus, andA. myosuroides, and the tetraploidA. pratensis. B chromosomes have been found in two species,A. pratensis andA. myosuroides and also in inter-specific hybrids betweenA. pratensis andA. arundinaceus. The characteristics, distribution and meiotic behaviour of both interchange heterozygotes and B chromosomes are described.     

Assessment of intergenomic recombination through GISH analysis of F1, BC1 and BC2 progenies of Tulipa gesneriana and T. fosteriana

Using 23 F1 hybrids, 14 BC1 and 32 BC2 progenies, the genome composition of Darwin hybrid tulips was analysed through genomic in situ hybridisation (GISH) of somatic chromosomes. All plants were diploids (2n = 2x = 24) with the exception of one tetraploid BC1 (2n = 4x = 48) and one aneuploid BC2 (2n = 2x + 1 = 25) hybrid. Morphometric analysis in F1 hybrids revealed a difference in the total length of chromosomes representing genomes of T. gesneriana and T. fosteriana, where the percentage of each genome equaled 55.18 ± 0.8 and 44.92 ± 0.6% respectively. GISH distinguished chromosomes from both parent genomes although there was a lack of consistent chromosome labelling in some cases. In both T. gesneriana and T. fosteriana chromosomes some segments of heterochromatin in the telomeric and intercalary regions exhibited a higher intensity of fluorescence. In situ hybridisation with 5S rDNA and 45S rDNA probes to metaphase chromosomes of F1 hybrids showed that these regions are rich in rDNA. A notable feature was that, despite genome differences, there was a considerable amount of intergenomic recombination between the parental chromosomes of the two species as estimated in both BC1 and BC2 offspring. The number of recombinant chromosomes ranged from 3 to 8 in BC1 and from 1 to 7 in BC2 progenies. All recombinant chromosomes possessed mostly a single recombinant segment derived from either a single crossover event or in a few cases double crossover events. This explains the fact that, unlike the situation in most F1 hybrids of other plant species, certain genotypes of Darwin hybrid tulips behave like normal diploid plants producing haploid gametes and give rise to mostly diploid sporophytes.     

The fate of recombinant chromosomes and genome interaction in Nicotiana asymmetric somatic hybrids and their sexual progeny

Genomic in-situ hybridization (GISH) was used to monitor the behaviour of parental genomes, and the fate of intergenomic chromosome translocations, through meiosis of plants regenerated from asymmetric somatic hybrids between Nicotiana sylvestris and N. plumbaginifolia. Meiotic pairing in the regenerants was exclusively between chromosomes or chromosome segments derived from the same species. Translocation (recombinant) chromosomes contained chromosome segments from both parental species, and were detected at all stages of meiosis. They occasionally paired with respectively homologous segments of N. sylvestris or N. plumbaginifolia chromosomes. Within hybrid nuclei, the meiotic division of N. plumbaginifolia lagged behind that of N. sylvestris. However, normal and recombinant chromosomes were eventually incorporated into dyads and tetrads, and the regenerants were partially pollen fertile. Recombinant chromosomes were transmitted through either male or female gametes, and were detected by GISH in sexual progeny obtained on selfing or backcrossing the regenerants to N. sylvestris. A new recombinant chromosome in one plant of the first backcross generation provided evidence of further chromosome rearrangements occurring at, or following, meiosis in the original regenerants. This study demonstrates the stable incorporation of chromosome segments from one parental genome of an asymmetric somatic hybrid into another, via intergenomic translocation, and reveals their transmission to subsequent sexual progeny.     

Chromosome association and pollen fertility of parental and interspecific hybrids of Lycopersicon esculentum X L. hirsutum and L. pennellii

Cytological studies were carried out on two wild species (L. hirsutum and L. pennellii) and the cultivated species (L. esculentum) of tomato and their F1 hybrids. Both parents and hybrids show a diploid chromosome number of 2n=24. The meiotic behaviour of the cultivated species showed a high degree of chromosome homology resulting in a high level of chiasmata frequency per bivalent. In contrast, the two wild species showed a slight increase in uniyalent frequency and a decrease in bivalent formation and chiasmata frequency. The meiotic behaviour of the hybrids showed a high level of univalents and low levels of bivalents as well as trivalents. Highly significant decreases in chiasmata frequency and increases in meiotic abnormalities, especially in the L. esculentum X L. pennellii hybrid, also were detected. The high meiotic irregularity and low chiasmata frequency recorded in the second hybrid indicated the disharmony and difference between its parental genomes and also served to predict its sterility. With regard to degree of pairing recorded in the hybrids, there is a possibility that sterility in such cases may refer to genetic factors in addition to the previously mentioned reasons. Pollen fertility showed no great difference between L. esculentum and L. hirsutum and their F1 hybrid, but a significant decrease was recorded in the L. esculentum X L. pennellii hybrid, which was clearly associated with high meiotic irregularity, low chiasmata frequency and chromosome association.