XX.—Cytological Studies of Melaniidæ (Mollusca) with Special Reference to Parthenogenesis and Polyploidy. II. A Study of Meiosis in the Rare Males of the Polyploid Race ofMelanoides tuberculatusandMelanoides lineatus

Abstract

In the rare males of the polyploid race of Melanoides tuberculatus, which constitute about 3 per cent of the population, spermatogenesis has been studied in detail and compared with certain stages of spermatogenesis in the rare males of M. lineatus, which constitute 0·01 per cent of the population. The occurrence of the rare males in these parthenogenetic Melanoides provides an exceptional opportunity for studying the pairing behaviour of the chromosomes and for obtaining a better understanding of the probable nature of the polyploidy involved. During spermatogenesis, the two usual meiotic divisions occur, but the different stages show univalents, bivalents, quadrivalents and bridge fragment configurations. Degeneration of sex cells occurs at different stages, but mostly at the spermatid stage and the sterility of the rare males may be said to be chromosomal. The significance of the various abnormal meiotic phenomena is discussed and it is suggested that autoallopolyploidy is involved. A conjectural scheme of the possible origin of the autoallopolyploid M. tuberculatus is also suggested and the interrelationships between parthenogenesis, polyploidy and hybridity discussed.