Low variability in a Y-linked plant gene and its implications for Y-chromosome evolution

Sex chromosomes have evolved independently in several different groups of organisms, but they share common features, including genetic degeneration of the Y chromosome. Suppression of recombination between ancestral proto-X and proto-Y chromosomes is thought to have led to their gradual divergence, and to degeneration of the Y chromosome, but the evolutionary forces responsible are unknown. In non-recombining Y chromosomes, deleterious mutations may be carried to fixation by linked advantageous mutations ("selective sweeps"). Occurrence of deleterious mutations may drive "Muller's ratchet" (stochastic loss of chromosomes with the fewest mutations). Selective elimination of deleterious mutations, causing "background selection" may accelerate stochastic fixation of mildly detrimental mutations. All these processes lower effective population sizes, and therefore reduce variability of genes in evolving Y chromosomes. We have studied DNA diversity and divergence in a recently described X- and Y-linked gene pair (SLX-1 and SLY-1) of the plant Silene latifolia to obtain evidence about the early stages of Y degeneration. Here we show that DNA polymorphism in SLY-1 is 20-fold lower than in SLX-1, but the pattern of polymorphism does not suggest a selective sweep.     

Annotated checklist and distribution of Culicoides biting midges of Ukraine (Diptera: Ceratopogonidae)

Despite the great socio-economic impact of Culicoides-borne diseases, there is little work on the reassessment of the faunistic and distributional data on this important group of vectors in the territory of Ukraine. Along with the literature review, materials were collected and slide-mounted using the conventional techniques for this study. The revised and commented list of 55 valid species names of the genus Culicoides recorded from Ukraine and their distribution is presented. New collection localities are given for Culicoides griseidorsum Kieffer, Culicoides atripennis Shevchenko and Culicoides newsteadi s.l. Culicoides triangulatus Shevchenko and Culicoides ukrainensis Shevchenko described from Ukraine are recognized as nomina dubia, whereas Culicoides tentorius Austen and Culicoides furcillatus Callot, Kremer and Paradis were excluded from the list. The current state of knowledge on Ukrainian Culicoides fauna should be characterized as rather understudied. Collectively, this suggests that further research efforts are needed, especially in view of the fact that the species recognized as vectors of the economically important diseases of animals are among the most abundant representatives of the genus. Moreover, the article highlights taxonomic problems within certain taxa and provides an outline of its further research perspectives.