The Evolution of Male Haploidy

Abstract

Among invertebrates of 7 different groups, a method of reproduction has been established in which parthenogenetically developed [male] [male] are haploid. Five problems involved in the attainment of this condition are pointed out and discussed. The hypothesis of Schrader and Hughes-Schrader of the gradual degeneration of one set of chromosomes to arrive at a haploid condition, is questioned. It is considered that their postulated stages in chromosome degeneration are independent phenomena. A review is given of some recently published cases of tychopar-thenogenesis, and of fixed parthenogenesis of unisexual ( [female] ) races and spp. As a possible explanation of the origin of d haploidy, a hypothesis is suggested of hybridization between a normal bisexual race and a parthenogenetic [female] race in which chromosomes have become modified. Among the segregating types resulting from this hybridization there should be azygous males. Nucleoplasmic regulation in these [male][male] should be gradually modified by an increase in the size of chromosomes, so that the haploid number might be retained into progressively later embryonic stages, especially in the germ track. In this way meiosis should be changed from the diploid to the haploid type.