First report on a multiple sex chromosome system (X<sub>1</sub>X<sub>2</sub>X<sub>3</sub>0) and population variations in the frequency of ring bivalents in Pyrrhocoridae (Hemiptera: Heteroptera)

Abstract

The family Pyrrhocoridae (Hemiptera: Heteroptera) is characterized by a modal diploid chromosome number of 16 (♂) rang- ing from 12 to 33 and simple (X0), multiple (X 1 X 2 0) and neo-sex chromosome systems (neoX-neoY). Out of about 340 known species, only 22 species belonging to 7 genera have to date been cytogenetically analysed. In the present study, the chromosome complement and meiotic details of one species, Odontopus nigricornis Stal has been revised, whilst that of another species, Antilochus russus Stal has been cytogenetically analyzed for the first time. The diploid chromosome complement of O. nigricornis is 2n (♂) = 25 = 22A + X 1 X 2 X 3 0; the first time this particular multiple sex chromosome system has been reported in the Pyrrhocoridae. Three sex chromosomes mostly remain intimately associated during male meiosis and their number was confirmed at diplotene and anaphase II, where they dissociate slightly to become distinct. Meiosis is post-reductional for sex chromosomes. However, unlike other pyrrhocorids with mul- tiple X chromosomes, the X 1 , X 2 and X 3 lie outside the autosomal ring on the metaphase plate during both divisions. The male diploid chromosome complement of A. russus was found to be 2n = 27 = 26A + X0. Apart from the typical meiotic features of heteropterans, the latter species shows inter- and intra-population variations in frequency of ring bivalents.