Studies of Gryllid Chromosomes II. Chromosomal polymorphisms in Pteronemobius taprobanensis (Walk.), and chromosome morphology of Loxoblemmus sp.

Abstract

1. Cytological examinations of 417 males of Pteronemobius taprobanensis and two males of Loxoblemmus sp. have been made. 2. The spermatogonial chromosome number in all the individuals of P. taprobanensis is 15 although this species is chromosomally polymorphic with respect to chromosomes No. 4 and 6 in order of size (vide infra). 3. The behaviour of the chromosomes in homozygous individuals of P. taprobanensis is regular and of orthodox pattern. There is some loss of chiasmata from diplotene to diakinesis stage. First meiotic metaphase chromosomes consist of 7 bivalents which could be numbered as 1-7 in order of shape and size, and the univalent X-chromosome. First two bivalents almost always are ring-shaped while bivalents No. 4 and 6 appear to be heteromorphic in heterozygous individuals. 4. In heterozygous individuals of P. taprobanensis, at metaphase I, bivalents No. 4 and 6 appear as hook-shaped structure formed of a J-shaped and a rod-shaped chromosomes. The behaviour of the heteromorphic bivalents before metaphase I could be followed occasionally. Out of 417 males examined, 162 individuals are heterozygous for chromosome 4, 69 for chromosome 6 and 78 both for the chromosomes 4 and 6. 5. The origin of the chromosomal polymorphism of P. taprobanensis has been discussed. Further, since 309 out of 417 males examined are heterozygous (about 74%), the heterozygotes are likely to enjoy a selective advantage and the species is under genetical fixation. 6. The diploid chromosome number of Loxoblemmus sp. is 13. A comparison of the caryotypes of this species with that of L. arietulus has been made. The chromosomes of Loxoblemmus sp. is different from other species firstly for the relatively small size of the X-chromosome and secondly for the number of V- and rod-shaped autosomes.