Heteromorphic Chromosome Pair Research Articles

Integrative morphological, cytogenetic and molecular characterization of the Andean climbing catfish Astroblepus mindoensis (Regan, 1916) (Siluriformes:Astroblepidae).

Astroblepus species, commonly known as Andean climbing catfish, exhibit a unique challenge in species delimitation, leading to ongoing taxonomic debates. Here we report data on Astroblepus mindoensis, a vulnerable species endemic to Ecuador, obtained by an integrative approach that includes cytogenetic analysis, molecular identification of the specimens, and recording of morphological and morphometric characters useful for species diagnosis. Thus, this study aimed to associate the karyotype data of the specimens analyzed with morphological and molecular characters, improving and expanding the existing taxonomic information, thus contributing to the systematics of the species. Our morphology results, unlike Regan's original description, which is brief and ambiguous, provide a more detailed morphometric and meristic description. Molecular phylogenetic reconstruction and genetic distance based on a fragment of the cytochrome c oxidase subunit I (COI) showed that our samples constitute a well-supported and monophyletic clade within the A. grixalvii species complex. The cytogenetic analysis identified distinct chromosomal markers, including a single cluster of major ribosomal genes (on chromosome pair 3) and of minor ribosomal genes (on chromosome pair 12) with their localization differing from those reported in other Astroblepus species analyzed. Additionally, the presence of a heteromorphic chromosome pair in males suggests the presence of an XX/XY sex-determination system that has not been identified in other congeneric species. Further investigation is necessary to determine if these chromosomes are associated with the accumulation of repeated sequences, as typically occurs with sex chromosomes, and to assess their presence in other species of the genus.

The First Identification of Homomorphic XY Sex Chromosomes by Integrating Cytogenetic and Transcriptomic Approaches in Plestiodon elegans (Scincidae)

The sex chromosomes of skinks are usually poorly differentiated and hardly distinguished by cytogenetic methods. Therefore, identifying sex chromosomes in species lacking easily recognizable heteromorphic sex chromosomes is necessary to fully understand sex chromosome diversity. In this paper, we employed cytogenetics, sex quantification of genes, and transcriptomic approaches to characterize the sex chromosomes in Plestiodon elegans. Cytogenetic examination of metaphases revealed a diploid number of 2n = 26, consisting of 12 macrochromosomes and 14 microchromosomes, with no significant heteromorphic chromosome pairs, speculating that the sex chromosomes may be homomorphic or poorly differentiated. The results of the sex quantification of genes showed that Calumenin (calu), COPI coat complex subunit γ 2 (copg2), and Smoothened (smo) were at half the dose in males as in females, suggesting that they are on the X chromosome. Transcriptomic data analysis from the gonads yielded the excess expression male-specific genes (n = 16), in which five PCR molecular markers were developed. Restricting the observed heterozygosity to males suggests the presence of homomorphic sex chromosomes in P. elegans, XX/XY. This is the first breakthrough in the study of the sex chromosomes of Plestiodon.

First Cytogenetic Analysis of Hemidactylus mercatorius Gray, 1842 Provides Insights on Interspecific Chromosomal Diversification in the Genus Hemidactylus (Squamata: Gekkonidae).

This contribution provides the first karyotype description of Hemidactylus mercatorius and discusses the interspecific chromosome diversification in the genus. Chromosomal analysis was performed on samples from different Malagasy populations using standard karyotyping, Ag-NOR staining, and banding methods (sequential C-banding + Giemsa, + Chromomycin A3, +4',6-diamidino-2-phenylindole). Irrespective of sex or sampling locality, H. mercatorius shows a karyotype of 2n = 42 with metacentric (1, 18-21), submetacentric (4), subtelocentric (5, 11), and acrocentric pairs (all the remaining pairs). There was no heteromorphic chromosome pair and no clear distinction between macro- and microchromosomes. NORs were localised close to the centromeres of a medium acrocentric pair (14). Heterochromatic blocks were identified on the telomeric and centromeric regions of most chromosome pairs. A comparison with the karyotype of H. mabouia highlights that the different morphology of several chromosome pairs clearly distinguishes the two species, contrasting the previously proposed synonymy. The differences between the karyotypes of H. mercatorius and H. mabouia concern the number of biarmed and acrocentric elements, suggesting the occurrence of several chromosome inversions. Considering all the available karyotype data on Hemidactylus and its sister genus Cyrtodactylus, it is possible to advance an evolutionary hypothesis on their chromosomal evolution, starting from a common ancestor with 2n = 48 and all acrocentric elements. From this ancestral condition, the karyotype diversification in the two genera has been prevalently characterised by a progressive accumulation of fusions and inversions which have reduced the total chromosome count and increased the number of biarmed chromosomes.

Cytogenetic Characteristics of Gerbillus dasyurus and Meriones tristrami (Rodentia: Gerbillinae) from Kilis, Turkey: Conventional and C Banded Karyotypes

In this study, diploid chromosome number and constitutive heterochromatin distributions of chromosomes were determined in Gerbillus dasyurus and Meriones tristrami species. The diploid chromosome numbers (2n) and autosomal chromosome arms (NFa) of G. dasyurus and M. tristrami were determined as 2n=60, NFa= 68, and 2n=72, NFa= 73, respectively. It was found that there were differences in autosomal chromosomal arms (NFa) of two different gerbil species, the karyotypes of which were obtained in this study, compared to previously conducted studies in Türkiye. Heteromorphic chromosome pair (submetacentric/acrocentric) was found in the autosomal set of M. tristrami. There was an enlarged heterochromatin block on the short arm of submetacentric chromosome in heteromorphic chromosome pair.

Karyotypes and ZZ/ZW Sex Chromosome System of Endemic Squalius recurvirostris (Leuciscinae, Cyprinidae) in Turkey

In this study, detailed cytogenetic properties of the endemic Akşehir chub, Squalius recurvirostris, distributed in the basin of the Ilgın (Çavuşçu) Lake basin was investigated by applying the standard Giemsa staining, C-banding and Ag-NOR technique. Diploid chromosome number is (2n) 50. A pair of heteromorphic chromosomes was detected in the karyotype in females. However, this difference was not observed in males. Therefore, it was determined that the heteromorphic chromosome pair in females were ZZ/ZW sex chromosomes. The Z chromosome in females is large acrocentric, and the W chromosome is medium-sized submeta/subtelocentric. The number of all chromosomal arms (NF) is 90, and the fundamental number of autosomal arms (NFa) is 88 in the karyotyped of the individuals studied. Some bi-armed and acrocentric chromosomes have centromeric C-bands in the chromosome set. The slightly centromeric or pericentromeric C-bands are observed in some chromosomes, while other chromosome pairs are C-negative. In the individuals studied two active NORs were detected. Both of these NORs were hemizygous and associated with a large C-heterochromatin region. One of the NORs was detected on the entire long arm of the submetacentric chromosome pair and the other on the entire short arm of another submetacentric chromosome pair.

Recombination landscape dimorphism and sex chromosome evolution in the dioecious plant Rumex hastatulus.

There is growing evidence from diverse taxa for sex differences in the genomic landscape of recombination, but the causes and consequences of these differences remain poorly understood. Strong recombination landscape dimorphism between the sexes could have important implications for the dynamics of sex chromosome evolution because low recombination in the heterogametic sex can favour the spread of sexually antagonistic alleles. Here, we present a sex-specific linkage map and revised genome assembly of Rumex hastatulus and provide the first evidence and characterization of sex differences in recombination landscape in a dioecious plant. We present data on significant sex differences in recombination, with regions of very low recombination in males covering over half of the genome. This pattern is evident on both sex chromosomes and autosomes, suggesting that pre-existing differences in recombination may have contributed to sex chromosome formation and divergence. Our analysis of segregation distortion suggests that haploid selection due to pollen competition occurs disproportionately in regions with low male recombination. We hypothesize that sex differences in the recombination landscape have contributed to the formation of a large heteromorphic pair of sex chromosomes in R. hastatulus, but more comparative analyses of recombination will be important to investigate this hypothesis further. This article is part of the theme issue 'Sex determination and sex chromosome evolution in land plants'.

Sex-biased dynamics of three-spined stickleback (Gasterosteus aculeatus) gene expression patterns

The study of the differences between sexes presents an excellent model to unravel how phenotypic variation is achieved from a similar genetic background. Sticklebacks are of particular interest since evidence of a heteromorphic chromosome pair has not always been detected. The present study investigated sex-biased mRNA and small non-coding RNA (sncRNA) expression patterns in the brain, adipose tissues, and gonads of the three-spined stickleback. The sncRNA analysis indicated that regulatory functions occurred mainly in the gonads. Alleged miRNA-mRNA interactions were established and a mapping bias of differential expressed transcripts towards chromosome 19 was observed. Key players previously shown to control sex determination and differentiation in other fish species but also genes like gapdh were among the transcripts identified. This is the first report in the three-spined stickleback demonstrating tissue-specific expression comprising both mRNA and sncRNA between sexes, emphasizing the importance of mRNA-miRNA interactions as well as new presumed genes not yet identified to have gender-specific roles.

New Data on Comparative Cytogenetics of the Mouse-Like Hamsters (Calomyscus Thomas, 1905) from Iran and Turkmenistan.

The taxonomy of the genus Calomyscus remains controversial. According to the latest systematics the genus includes eight species with great karyotypic variation. Here, we studied karyotypes of 14 Calomyscus individuals from different regions of Iran and Turkmenistan using a new set of chromosome painting probes from a Calomyscus sp. male (2n = 46, XY; Shahr-e-Kord-Soreshjan-Cheshme Maiak Province). We showed the retention of large syntenic blocks in karyotypes of individuals with identical chromosome numbers. The only rearrangement (fusion 2/21) differentiated Calomyscus elburzensis, Calomyscus mystax mystax, and Calomyscus sp. from Isfahan Province with 2n = 44 from karyotypes of C. bailwardi, Calomyscus sp. from Shahr-e-Kord, Chahar Mahal and Bakhtiari-Aloni, and Khuzestan-Izeh Provinces with 2n = 46. The individuals from Shahdad tunnel, Kerman Province with 2n = 51–52 demonstrated non-centric fissions of chromosomes 4, 5, and 6 of the 46-chromosomal form with the formation of separate small acrocentrics. A heteromorphic pair of chromosomes in a specimen with 2n = 51 resulted from a fusion of two autosomes. C-banding and chromomycin A3-DAPI staining after G-banding showed extensive heterochromatin variation between individuals.

The Pontastacus leptodactylus (Astacidae) Repeatome Provides Insight Into Genome Evolution and Reveals Remarkable Diversity of Satellite DNA.

Pontastacus leptodactylus is a native European crayfish species found in both freshwater and brackish environments. It has commercial importance for fisheries and aquaculture industries. Up till now, most studies concerning P. leptodactylus have focused onto gaining knowledge about its phylogeny and population genetics. However, little is known about the chromosomal evolution and genome organization of this species. Therefore, we performed clustering analysis of a low coverage genomic dataset to identify and characterize repetitive DNA in the P. leptodactylus genome. In addition, the karyogram of P. leptodactylus (2n = 180) is presented here for the first time consisting of 75 metacentric, 14 submetacentric, and a submetacentric/metacentric heteromorphic chromosome pair. We determined the genome size to be at ~18.7 gigabase pairs. Repetitive DNA represents about 54.85% of the genome. Satellite DNA repeats are the most abundant type of repetitive DNA, making up to ~28% of the total amount of repetitive elements, followed by the Ty3/Gypsy retroelements (~15%). Our study established a surprisingly high diversity of satellite repeats in P. leptodactylus. The genome of P. leptodactylus is by far the most satellite-rich genome discovered to date with 258 satellite families described. Of the five mapped satellite DNA families on chromosomes, PlSAT3-411 co-localizes with the AT-rich DAPI positive probable (peri)centromeric heterochromatin on all chromosomes, while PlSAT14-79 co-localizes with the AT-rich DAPI positive (peri)centromeric heterochromatin on one chromosome and is also located subterminally and intercalary on some chromosomes. PlSAT1-21 is located intercalary in the vicinity of the (peri)centromeric heterochromatin on some chromosomes, while PlSAT6-70 and PlSAT7-134 are located intercalary on some P. leptodactylus chromosomes. The FISH results reveal amplification of interstitial telomeric repeats (ITRs) in P. leptodactylus. The prevalence of repetitive elements, especially the satellite DNA repeats, may have provided a driving force for the evolution of the P. leptodactylus genome.

Supergene, sex and sociality

Intraspecific variability in social organization is common, yet the underlying causes are rarely known. I will show that the existence of two divergent forms of social organization in six ant species is under the control of a pair of heteromorphic chromosomes that have many of the key properties of sex chromosomes. In particular, this social chromosome contains a large (13 megabases) region in which recombination is completely suppressed via three large inversions ( Fig. 1 ). These findings highlight how genomic rearrangements can maintain divergent adaptive social phenotypes involving many genes acting together by locally limiting recombination.

Karyotype structure and NOR activity in Brazilian Smilax Linnaeus, 1753 species (Smilacaceae).

The genus Smilax Linnaeus, 1753 (Smilacaceae) is a large genus of dioecious plants distributed in tropical, subtropical and temperate regions. Some Smilax species have medicinal importance and their identification is important for the control of raw material used in the manufacture of phytotherapeutical products. The karyotypes of seven Brazilian Smilax species were investigated. Mitotic metaphases of roots from young plants were analysed in Feulgen-stained preparations. The karyotypes were asymmetric and modal with 2n = 2x = 32 chromosomes gradually decreasing in size. In S. goyazana A De Candolle & C De Candolle, 1878, a polyploid species, 2n = 4x = 64. In all the species, the large and medium-sized chromosomes were subtelocentric and submetacentric and the small chromosomes were submetacentric or metacentric. Their karyotypes were quite similar, with differences in the arm ratio of some chromosomes. S. fluminensis Steudel, 1841 differed from the other species by having a large metacentric chromosome 1. These findings suggest that evolution occurred without drastic changes in the chromosomal structure in the species analyzed. Terminal secondary constrictions were visualized on the short arm of some chromosomes, but they were detected only in one homologue of each pair. Due to the terminal location and the degree of chromosome condensation, secondary constrictions were not visualized in some species. The nucleolus organizer regions (NORs) were mapped by silver-staining and fluorescent in situ hybridization (FISH) in S. rufescens Grisebach, 1842 and S. fluminensis. Silver-staining and FISH signals were colocalized on the short arms of six chromosomes in S. rufescens and four chromosomes in S. fluminensis. In FISH preparations, one of the largest chromosomes had the secondary constrictions highly decondensed in some cells. This finding and the heteromorphism observed in Feulgen-stained chromosomes suggest that differential rRNA gene expression between homologous rDNA loci can occur in some cells, resulting in different degrees of ribosomal chromatin decondensation. The presence of a heteromorphic chromosome pair in S. rufescens, S. polyantha Grisebach, 1842 and S. goyazana suggests a chromosomal sex determination in these dioecious species.

The chromosomes of Drosophila suzukii (Diptera: Drosophilidae): detailed photographic polytene chromosomal maps and in situ hybridization data

The spotted wing drosophila, D. suzukii, is a serious agricultural pest attacking a variety of soft fruits and vegetables. Although originating from East Asia it has recently invaded America and Europe raising major concern about its expansion potential and the consequent economic losses. Since cytogenetic information on the species is scarce, we report here the mitotic karyotype and detailed photographic maps of the salivary gland polytene chromosomes of D. suzukii. The mitotic metaphase complement contains three pairs of autosomes, one of which is dot-like, and one pair of heteromorphic (XX/XY) sex chromosomes. The salivary gland polytene complement consists of five long polytene arms, representing the two metacentric autosomes and the acrocentric X chromosome, and one very short polytene element, which corresponds to the dot-like autosome. Banding pattern as well as the most characteristic features and prominent landmarks of each polytene chromosome arm are presented and discussed. Furthermore, twelve gene markers have been mapped on the polytene chromosomes of D. suzukii by in situ hybridization. Their distribution pattern was found quite similar to that of D. melanogaster revealing conservation of synteny although the relative position within each chromosome arm for most of the genes differed significantly between D. suzukii and D. melanogaster. The chromosome information presented here is suitable for comparative cytogenetic studies and phylogenetic exploration, while it could also facilitate the assembly of the genome sequence and support the development of genetic tools for species-specific and environment-friendly biological control applications such as the sterile insect technique.

Karyotype Analysis of Four Blind Snake Species (Reptilia: Squamata: Scolecophidia) and Karyotypic Changes in Serpentes

The suborder Serpentes is divided into 2 infraorders, Scolecophidia and Alethinophidia, which diverged at an early stage of snake diversification. In this study, we examined karyotypes of 4 scolecophidian species (Letheobia simonii, Xerotyphlops vermicularis, Indotyphlops braminus, and Myriopholis macrorhyncha) and performed FISH with 18S-28S rDNA as well as microchromosomal and Z chromosome-linked genes of Elaphe quadrivirgata (Alethinophidia) to investigate the karyotype evolution in the scolecophidian lineage. Diploid chromosome numbers of X. vermicularis and L. simonii were 30 (16 macrochromosomes and 14 microchromosomes) and 32 (16 macrochromosomes and 16 microchromosomes), respectively. The karyotype of a female M. macrorhyncha consisted of 15 macrochromosomes and 19 microchromosomes, including a heterochromatic microchromosome, indicating the presence of a heteromorphic chromosome pair. E. quadrivirgata Z-linked genes mapped to chromosome 4 of M. macrorhyncha, not to the heteromorphic pair. Therefore, M. macrorhyncha may have differentiated ZW sex chromosomes which are not homologous to those of E. quadrivirgata. One of the E. quadrivirgata microchromosomal genes mapped to the terminal region of chromosome 4q in X. vermicularis, suggesting that fusions between microchromosomes and macrochromosomes occurred in this species. rDNA was localized in different macrochromosomal pairs in the 2 diploid scolecophidian snakes examined here, whereas the gene location in a microchromosomal pair was conserved in 5 alethinophidian species examined. These results might imply the occurrence of chromosome fusions in the scolecophidian lineages. In I. braminus, a unique parthenogenetic snake with a triploid karyotype (21 macrochromosomes and 21 microchromosomes), morphological heteromorphisms were identified in chromosomes 1 and 7. Such heteromorphisms in 2 chromosomes were also observed in individuals from distant locations in the broad distribution range of this species, suggesting that the heteromorphisms were fixed in the genome at an early stage of its speciation.

Degenerative Expansion of a Young Supergene.

Long-term suppression of recombination ultimately leads to gene loss, as demonstrated by the depauperate Y and W chromosomes of long-established pairs of XY and ZW chromosomes. The young social supergene of the Solenopsis invicta red fire ant provides a powerful system to examine the effects of suppressed recombination over a shorter timescale. The two variants of this supergene are carried by a pair of heteromorphic chromosomes, referred to as the social B and social b (SB and Sb) chromosomes. The Sb variant of this supergene changes colony social organization and has an inheritance pattern similar to a Y or W chromosome because it is unable to recombine. We used high-resolution optical mapping, k-mer distribution analysis, and quantification of repetitive elements on haploid ants carrying alternate variants of this young supergene region. We find that instead of shrinking, the Sb variant of the supergene has increased in length by more than 30%. Surprisingly, only a portion of this length increase is due to consistent increases in the frequency of particular classes of repetitive elements. Instead, haplotypes of this supergene variant differ dramatically in the amounts of other repetitive elements, indicating that the accumulation of repetitive elements is a heterogeneous and dynamic process. This is the first comprehensive demonstration of degenerative expansion in an animal and shows that it occurs through nonlinear processes during the early evolution of a region of suppressed recombination.

Cytogenetics of Two Onychostoma Species in Taiwan by Ag-NOR and 18S rDNA Profiles.

Chiao-Chuan Han, Tsair-Bor Yen, Nian-Cih Chen, and Mei-Chen Tseng (2017) Both Onychostoma barbatulum and O. alticorpus are primary freshwater fish in Taiwan. The former has been developed as an aquaculture species with high economic value, while the latter is a native endemic species in Taiwan. Understanding the cytogenetic information of these two species is necessary for their selected breeding, recovery, and management. In this study, Giemsa staining, silver-binding nucleolar organizer region (Ag-NOR), C-banding, and fluorescence in situ hybridization (FISH) with 18S ribosomal (r)DNA probes were used to analyze the cytogenetic characteristics. Results of Giemsa staining showed that the two Onychostoma species shared the same number of chromosomes, 2n = 50. Respective karyotype formulas of the female and male were 10 m + 22 sm + 10 st + 8 t and 11 m + 22 sm + 10 st + 7 t in O. barbatulum, and 14 m + 18 sm + 8 st + 10 t and 15 m + 18 sm + 8 st + 9 t in O. alticorpus. Karyotypes of both species showed a pair of heteromorphic chromosomes in male fish. Their sex determination should be the XX/XY system. Two pairs of Ag-NORs were found in O. barbatulum, but only one pair occurred in O. alticorpus. C-banding areas were observed on centromeres or telomeres of some chromosomes. FISH revealed different cytogenetic characters between these two species. The above cytogenetic information will contribute to species identification, population recovery, and advantages for breeding and management in the future.

Karyological analysis and NOR polymorphism of phayre’s langur, Trachypithecus phayrei crepuscula (Primate, Colobinae) in Thailand

Karyological analysis of Phayre’s langur (Trachypithecus phayrei crepuscula) at Songkhla Zoo, Thailand has been reported in this present article. Conventional staining, GTG-banding, and high-resolution technical analysis were carried out on standard whole blood T-lymphocyte culture from 4 specimens of T. phayrei crepuscula. The results showed that the diploid number is 2n = 44 and the fundamental number is 88 in both male and female. The autosomes consisted of 6 large metacentric, 10 large submetacentric, 2 large acrocentric, 6 medium metacentric, 14 medium submetacentric, 2 small submetacentric, and 2 small acrocentric chromosomes. The X chromosome is the large submetacentric chromosome, while the Y chromosome is medium metacentric chromosome. From GTG-banding and using high-resolution techniques, the numbers of bands and locations in T. phayrei crepuscula are 224 and 259, respectively, and each chromosome pair could be clearly differentiated. In addition, the long arm of subcentromeric chromosome pair 17 showed clearly observable nucleolar organizer regions/NORs. This is the first report on polymorphism of NORs in T. phayrei crepuscula. Thus the results indicated that a heteromorphic chromosome pair with different sizes of NORs is present for chromosome pair 17 (17a 17b) in both male and female animal. The results provide useful information for further cytogenetic study of this species. The karyotype formula could be deduced as: $$2n\left( {44} \right) \, = {\text{ L}}_{6}^{\text{m}} + {\text{L}}_{10}^{\text{sm}} + {\text{L}}_{2}^{\text{a}} + {\text{M}}_{6}^{\text{m}} + {\text{M}}_{14}^{\text{sm}} + {\text{S}}_{2}^{\text{sm}} + {\text{S}}_{2}^{\text{a}} + 2 \,$$ Sex-chromosomes.

When can chromosomes drive speciation? The peculiar case of the Malagasy tomato frogs (genus Dyscophus)

Except for polyploidy, parthenogenesis and hybridogenesis, the role of chromosome rearrangements in speciation is still highly debated. This paper highlights an unusual karyotype diversification and a putative case of chromosomal speciation in the endemic Malagasy tomato frogs.The tomato frogs include Dyscophus insularis as well as the sister species D. antongilii and D. guineti, which are characterized by a disjunct distribution and a low genetic divergence, probably due to a recent hybridization. Our results show that the three Dyscophus species present 2n=26 biarmed chromosomes, differing in chromatin pattern. Interestingly, both sexes of D. guineti show a heteromorphic chromosome pair 11, with one element larger (11a) than the other (11b) due to the presence of a large terminal tract on the short arm with an euchromatic band enclosed between two heterochromatic bands. This fixed condition reminds that of some newts of the genus Triturus, but in Dyscophus it probably characterizes the beginning of a cladogenetic process. We hypothesize that in D. guineti the raising of the heteromorphic 11th chromosome pair followed the hybridization with D. antongilii, originating a post-zygotic barrier and representing a case of sympatric or parapatric chromosomal speciation. This speciation mechanism may also have had important biogeographic consequences, shaping the current disjunct distribution of D. antongilii and D. guineti, so far not sufficiently explained.

Comparative karyotype investigations in the white-clawed crayfish Austropotamobius pallipes (Lereboullet, 1858) species complex and stone crayfish A. torrentium (Schrank, 1803) (Decapoda: Astacidae)

We report for the first time the chromosome number and karyological characteristics of the endangered European freshwater crayfish genus Austropotamobius (Decapoda, Astacidae). The karyotypes of the white-clawed crayfish ( Austropotamobius pallipes species complex) and of the stone crayfish ( A. torrentium ) consisted of 2 n = 176 chromosomes arranged into 76 metacentric, 11 metacentric-submetacentric, and one heteromorphic acrocentric/metacentric chromosome pair. Fluorochrome staining with 4,6-diamino-2-phenylindole (DAPI) revealed distinctive centromeric/pericentromeric as well as interstitial AT-rich blocks in A. pallipes and A. torrentium . Centromeric/pericentromeric AT-rich blocks were observed on all chromosomes, whereas interstitial AT-rich blocks were observed on some chromosomes, mostly on the larger ones. Fluorescence in situ hybridization (FISH) with the 45S rDNA probe on mitotic metaphases of A. pallipes and A. torrentium have revealed four major Texas-red signals, corresponding to four sites of 45S rDNA. One of the two 45S rDNA-bearing chromosome pairs was highly heteromorphic, considering the length of chromosomes as well as the size and intensity of 45S rDNA FISH signal. We speculate that the heteromorphic chromosome pair might represents X and Y sex chromosomes, suggesting the presence of XX-XY sex determination system in the A. pallipes species complex and A. torrentium .

On Sexual Dimorphism Of Karyotypes Of Viviparus viviparus And V. contectus (Gastropoda, Viviparidae)

Abstract Karyotype analysis of males and females of two Viviparid species Viviparus viviparus Linnaeus, 1758 and V. contectus (Millet, 1813), shows differences in their karyotypes in the absence of heteromorphic pairs of chromosomes. This indicates an epigamous mechanism of sex determination.

Purifying Selection Maintains Dosage-Sensitive Genes during Degeneration of the Threespine Stickleback Y Chromosome.

Sex chromosomes are subject to unique evolutionary forces that cause suppression of recombination, leading to sequence degeneration and the formation of heteromorphic chromosome pairs (i.e., XY or ZW). Although progress has been made in characterizing the outcomes of these evolutionary processes on vertebrate sex chromosomes, it is still unclear how recombination suppression and sequence divergence typically occur and how gene dosage imbalances are resolved in the heterogametic sex. The threespine stickleback fish (Gasterosteus aculeatus) is a powerful model system to explore vertebrate sex chromosome evolution, as it possesses an XY sex chromosome pair at relatively early stages of differentiation. Using a combination of whole-genome and transcriptome sequencing, we characterized sequence evolution and gene expression across the sex chromosomes. We uncovered two distinct evolutionary strata that correspond with known structural rearrangements on the Y chromosome. In the oldest stratum, only a handful of genes remain, and these genes are under strong purifying selection. By comparing sex-linked gene expression with expression of autosomal orthologs in an outgroup, we show that dosage compensation has not evolved in threespine sticklebacks through upregulation of the X chromosome in males. Instead, in the oldest stratum, the genes that still possess a Y chromosome allele are enriched for genes predicted to be dosage sensitive in mammals and yeast. Our results suggest that dosage imbalances may have been avoided at haploinsufficient genes by retaining function of the Y chromosome allele through strong purifying selection.