Chromosome Studies in Pisces

IV. The chromosomes of Mogrunda obscura (Gobiidae), with evidence of male heterogamety

Abstract

The chromosomes of Mogrunda obscura (T. et S.), a species of the Gobiidae, were investigated in male germ cells throughout the course of spermatogenesis, with special reference to the sexual difference of the chromosomes. The diploid number of chromosomes was found to be 62 in the spermatogonium and the haploid number of 31 was observed in both primary and secondary spermatocytes. All the chromosomes are characterized by a simple rod-type of telomitic nature. Remarkable is the occurrence of the sex-chromosomes of the XY-mechanism found in this species.
The primary spermatocyte metaphase is provided with 30 bivalent chromosomes having ordinary structure and a heteromorphic bivalent which consists of an elongate rod-shaped one connected in its extremity with a small rod-element in an end-to-end series. The heteromorphic bivalent is very prominent by lying always in the periphery of the equatorial plate along with its asymmetrical feature. In the first division the heteromorphic bivalent disjoins into two unequal components, an elongated element and a small body. In relative magnitude the larger one is approximately twice the size of the smaller one. The first division produces always two different kinds of secondary spermatocytes, despite the equal number of chromosomes, in respect to these particular elements. The history of these two particular elements was followed as heteropycnotic bodies from their appearance in the early stage of the growing period of the spermatocyte until the metaphase of the first division. This series of facts makes it most probable that these unequal chromosomes are to be interpreted as the XY-complex, the larger one being the X and the smaller the Y. This report is the first to record the cytological evidence of male heterogamety in fishes.