Recent origin of a chiasmatic sex trivalent in Australian Pseudotetracha tiger beetles

Meiotic behavior based on observations of the first and second divisions was studied in males of four taxa of the Australian tiger beetle genus Pseudotetracha of the tribe Megacephalini (Coleoptera). Pseudotetracha blackburni clade 1 shows 10 pairs of autosomes plus a trivalent that is hypothesized to be the result of either a translocation or a fusion in which the original heterosomes (very likely XY) and an autosomal pair are involved, giving rise to a recently established neo-X1X2Y sex chromosome system of chiasmatic nature. The origin of this karyotype has been determined to have taken place 2.30–3.72 million years in the past using a molecular clock based on the 16S rRNA substitution rate. Pseudotetracha blackburni clade 2 shows a meioformula of the type n = 11 + XY, the same as that found in the related taxon P. australis. Previous data for P. whelani, with 12 pairs of autosomes and an XY sex chromosome system, are confirmed in this survey. The multiple chiasmatic sex chromosome system of P. blackburni clade 1 is considered to be of recent origin and with an evolutionarily short-life confined to this species, where close relatives exhibit simple genetic systems, in contrast to the long evolutionary life of the multiple achiasmatic sex chromosome system broadly found in the tribes Cicindelini and Collyrini. The implications of this chromosomal rearrangement in terms of recombination and speciation are discussed. The results of this work, together with the available cytogenetic data for other Megacephalini species, are interpreted in the light of recent molecular phylogenies of the tribe, showing evidence of a possible process of karyotypic orthoselection with recurrent cycles of incorporation of autosomes to the heterosome pair and subsequent loss of the Y chromosome in Tetracha and Pseudotetracha.