Cytogenetics and evolution of Simulium ornatipes skuse (Diptera:Simuliidae) I. Sibling speciation

Larval populations of the morphologically distinct species Simulium ornatipes and S. neornatipes and a subspecies of the former, S. ornatipes norfolkense, were analysed using polytene chromosome cytology. Fixed inversion differences, distinct differentiated sex chromosomes and unique arrays of chromosomal polymorphisms reveal two sibling species, S. ornatipes A and B, within S. ornatipes, and two, S. neornatipes 1 and 2, within S. neornatipes. Chromosomally S. ornatipes norfolkense is similar to S. ornatipes A. An unprecedented level of shared inversion polymorphism is shown by S. ornatipes A and B indicating a very close relationship between them. While it may be postulated that such a situation has arisen by independent origin of the shared inversions within each species, or by introgression of these rearrangements between the two species, it is concluded that the shared polymorphisms in fact originate from a common ancestor. Within the four chromosomally defined species, differentiated sex chromosome systems occur which may involve any of the three chromosome pairs. Simulium ornatipes A is particularly interesting because there are indications that it may be polymorphic for differentiated sex chromosomes on two different elements and that it may also share a sex chromosome marker with S. ornatipes B, a unique situation in the Simuliidae. The evolution of sex chromosome systems in the four species is compared with that of other Simulids and its intimate connection with speciation emphasised. Finally the data on fixed inversions, differentiated sex chromosomes and shared polymorphisms is used to construct a phylogeny of the four species outlining their patterns of speciation.     

Home range size in two darkling beetle species (Coleoptera,Tenebrionidae) from Southern Kazakhstan

The repeated collecting of marked darkling beetle species, including the diurnal species Tentyria gigas and the nocturnal species Cyphogenia gibba from Kazakhstan demonstrated that their home ranges constituted 457 and 360 m², respectively. More than 500 beetles from an area of 2.5 ha were marked by engraving numbers on their elytra. Beetles were observed during two seasons at a distance covering 2 km from the site of release. Sex was determined in a quarter of all the beetles. In females and males of T. gigas, the home range constituted 700 and 552 m², respectively; in C. gibba, 599 and 343 m², respectively. Small home ranges of these species can be explained by the presence of numerous predators, occasional death of beetles under cattle’s hoofs, and a great number and high density of shelters.     

Independent Origins of New Sex-Linked Chromosomes in the <Emphasis Type="Italic">melanica</Emphasis> and <Emphasis Type="Italic">robusta</Emphasis> Species Groups of <Emphasis Type="Italic">Drosophila</Emphasis>

Background  

Recent translocations of autosomal regions to the sex chromosomes represent important systems for identifying the evolutionary forces affecting convergent patterns of sex-chromosome heteromorphism. Additions to the sex chromosomes have been reported in the melanica and robusta species groups, two sister clades of Drosophila. The close relationship between these two species groups and the similarity of their rearranged karyotypes motivates this test of alternative hypotheses; the rearranged sex chromosomes in both groups are derived through a common origin, or the rearrangements are derived through at least two independent origins. Here we examine chromosomal arrangement in representatives of the melanica and the robusta species groups and test these alternative hypotheses using a phylogenetic approach.     

CONSERVED SEX CHROMOSOMES ACROSS ADAPTIVELY RADIATED ANOLIS LIZARDS

Vertebrates possess diverse sex‐determining systems, which differ in evolutionary stability among particular groups. It has been suggested that poikilotherms possess more frequent turnovers of sex chromosomes than homoiotherms, whose effective thermoregulation can prevent the emergence of the sex reversals induced by environmental temperature. Squamate reptiles used to be regarded as a group with an extensive variability in sex determination; however, we document how the rather old radiation of lizards from the genus Anolis, known for exceptional ecomorphological variability, was connected with stability in sex chromosomes. We found that 18 tested species, representing most of the phylogenetic diversity of the genus, share the gene content of their X chromosomes. Furthermore, we discovered homologous sex chromosomes in species of two genera (Sceloporus and Petrosaurus) from the family Phrynosomatidae, serving here as an outgroup to Anolis. We can conclude that the origin of sex chromosomes within iguanas largely predates the Anolis radiation and that the sex chromosomes of iguanas remained conserved for a significant part of their evolutionary history. Next to therian mammals and birds, Anolis lizards therefore represent another adaptively radiated amniote clade with conserved sex chromosomes. We argue that the evolutionary stability of sex‐determining systems may reflect an advanced stage of differentiation of sex chromosomes rather than thermoregulation strategy.     

Dynamic gene order on the <Emphasis Type="Italic">Silene latifolia</Emphasis> Y chromosome

Dioecious Silene latifolia evolved heteromorphic sex chromosomes within the last ten million years, making it a species of choice for studies of the early stages of sex chromosome evolution in plants. About a dozen genes have been isolated from its sex chromosomes and basic genetic and deletion maps exist for the X and Y chromosomes. However, discrepancies between Y chromosome maps led to the proposal that individual Y chromosomes may differ in gene order. Here, we use an alternative approach, with fluorescence in situ hybridization (FISH), to locate individual genes on S. latifolia sex chromosomes. We demonstrate that gene order on the Y chromosome differs between plants from two populations. We suggest that dynamic gene order may be a general property of Y chromosomes in species with XY systems, in view of recent work demonstrating that the gene order on the Y chromosomes of humans and chimpanzees are dramatically different.     

Single Origin of Sex Chromosomes and Multiple Origins of B Chromosomes in Fish Genus Characidium

Chromosome painting with DNA probes obtained from supernumerary (B) and sex chromosomes in three species of fish genus Characidium (C. gomesi, C. pterostictum and C. oiticicai) showed a close resemblance in repetitive DNA content between B and sex chromosomes in C. gomesi and C. pterostictum. This suggests an intraspecific origin for B chromosomes in these two species, probably deriving from sex chromosomes. In C. oiticicai, however, a DNA probe obtained from its B chromosome hybridized with the B but not with the A chromosomes, suggesting that the B chromosome in this species could have arisen interspecifically, although this hypothesis needs further investigation. A molecular phylogenetic analysis performed on nine Characidium species, with two mtDNA genes, showed that the presence of heteromorphic sex chromosomes in these species is a derived condition, and that their origin could have been unique, a conclusion also supported by interspecific chromosome painting with a CgW probe derived from the W chromosome in C. gomesi. Summing up, our results indicate that whereas heteromorphic sex chromosomes in the genus Characidium appear to have had a common and unique origin, B chromosomes may have had independent origins in different species. Our results also show that molecular phylogenetic analysis is an excellent complement for cytogenetic studies by unveiling the direction of evolutionary chromosome changes.     

Whole chromosome painting reveals independent origin of sex chromosomes in closely related forms of a fish species

The wolf fish Hoplias malabaricus includes well differentiated sex systems (XY and X₁X₂Y in karyomorphs B and D, respectively), a nascent XY pair (karyomorph C) and not recognized sex chromosomes (karyomorph A). We performed the evolutionary analysis of these sex chromosomes, using two X chromosome-specific probes derived by microdissection from the XY and X₁X₂Y sex systems. A putative-sex pair in karyomorph A was identified, from which the differentiated XY system was evolved, as well as the clearly evolutionary relationship between the nascent XY system and the origin of the multiple X₁X₂Y chromosomes. The lack of recognizable signals on the sex chromosomes after the reciprocal cross-FISH experiments highlighted that they evolved independently from non-homologous autosomal pairs. It is noteworthy that these distinct pathways occur inside the same nominal species, thus exposing the high plasticity of sex chromosome evolution in lower vertebrates. Possible mechanisms underlying this sex determination liability are also discussed.     

Is the Y chromosome of Drosophila an evolved supernumerary chromosome?

The Y chromosomes of most Drosophila species are necessary for male fertility but they are not involved in sex determination. They have many puzzling properties that resemble the effects caused by B chromosomes. Classical genetic and molecular studies reveal substantial affinities between Y and B chromosomes and suggest that the Y chromosomes of Drosophila are not degenerated homologues of the X chromosomes, but rather that their Y chromosomes evolved as specialized supernumeraries similar to classical B chromosomes.     

Dynamic sex chromosomes in Old World chameleons (Squamata: Chamaeleonidae)

Much of our current state of knowledge concerning sex chromosome evolution is based on a handful of ‘exceptional’ taxa with heteromorphic sex chromosomes. However, classifying the sex chromosome systems of additional species lacking easily identifiable, heteromorphic sex chromosomes is indispensable if we wish to fully understand the genesis, degeneration and turnover of vertebrate sex chromosomes. Squamate reptiles (lizards and snakes) are a potential model clade for studying sex chromosome evolution as they exhibit a suite of sex‐determining modes yet most species lack heteromorphic sex chromosomes. Only three (of 203) chameleon species have identified sex chromosome systems (all with female heterogamety, ZZ/ZW). This study uses a recently developed method to identify sex‐specific genetic markers from restriction site‐associated DNA sequence (RADseq) data, which enables the identification of sex chromosome systems in species lacking heteromorphic sex chromosomes. We used RADseq and subsequent PCR validation to identify an XX/XY sex chromosome system in the veiled chameleon (Chamaeleo calyptratus), revealing a novel transition in sex chromosome systems within the Chamaeleonidae. The sex‐specific genetic markers identified here will be essential in research focused on sex‐specific, comparative, functional and developmental evolutionary questions, further promoting C. calyptratus’ utility as an emerging model organism.     

On the sex chromosome trivalent in some Lepidoptera females

In four of the moth species investigated, viz. Witlesia murana, Scoparia arundinata (Pyraloidea), Bactra furfurana and B. lacteana (Tortricoidea) the metaphase plates of the first meiotic division of their oocytes show a trivalent in addition to the normal bivalents. It evidently has its rise in a transverse break in one of the conjugated chromosomes. Two sex chromatin bodies can be seen in the female somatic cells of three of these species, whereas other species with a normal XY bivalent have only one. These two sex chromatin bodies are unequal in size, and their sizes bear approximately the same relation to each other as do those of the two smaller chromosomes of the trivalent. The broken chromosome is evidently the Y chromosome. The sex chromosome designation for the four above-mentioned species is thus XY₁Y₂ for the females and XX for the males. The sex chromosomes of the four species are among the biggest of the respective complements. This supports the view that the big chromosome to be found in several Lepidoptera species is the sex chromosome. It seems that in animals with holokinetic chromosomes an excessive fragmentation is hindered, at least in the case of the sex chromosomes, by its deleterious effect on the balance of sex-determining genes.Dedicated to Doctor Sally Hughes-Schrader on the occasion of her seventy-fifth birthday.