Marked sex chromosomal variations between Indian and Japanese species of Physopelta (Largidae, Heteroptera)

Abstract

Testes of Physopelta schlanbuschi, P. quadrigutta and P. gutta studied from India contained 2n=17, 12A+2m+X1X2Y chromosomes in spermatogonial metaphase and 6 autosomal bivalents, two m chromosomes and 3 univalent sex chromosomes (X1X2Y) during prophase I and metaphase I. Anaphae I was reductional for the autosomes and equational for the sex chromosomes and at metaphase II sex chromosomes formed a pseudotrivalent structure located in the mid region and the autosomes formed a ring like arrangement in the equatorial region of the hollow spindle. Anaphase II was reductional for the sex chromosomes, for X1 and X2 moved to one pole and the Y to the opposite pole. The X1 and the Y were morphologically indistinguishable and formed two conspicuously large markers with some difference in their size in the 3 species among which P. gutta had relatively small X1 and Y. The Japanese population of P. gutta had the same chromosome constitution like that of the Indian population but the relative size of the X1 and Y in two populations varied and in Japanese population they were not readily demarcable from 12 small autosomes in spermatogonial plates. Males of another species, P. cincticollis also studied from Japan had 2n=15, 12A+2m+XO in spermatogonial complement and 6 autosomal bivalents, two m chromosomes and the single X at prophase I and metaphase I while 6 autosomes, one m chromosome and the X at the metaphase II. Anaphase II was reductional for the sex chromosome, the X moved to only one pole with 6 autosomes and one m while the opposite pole was without the X. Thus species of Physopelta studied from India had the uniform diploid number, sex chromosome mechanism and the meiotic behaviour of chromosomes while those of Japan had two types of diploid numbers, 15 in XO species and 17 in X1X2Y species. Thus Japanese species of Physopelta were chromosomally more variable showing intra- and interspecific sex chromosomal variations.The presence of Y chromosome in 3 out of 4 species of Physopelta happened to be exceptional as the same has not been recorded in any other species in Largidae and in its allied family Pyrrhocoridae so far. The occurrence of only X1X2Y type males in India might have selective advantage and different tentative suggestions have been put forth to account for the origin of the diversified sex chromoso memechanism in different species of Physopelta.