POLYPLOIDY IN MORMONIELLA

Abstract

F O R the Hymenoptera in general, chromosome number is haploid for males and diploid for females, the former developing parthenogenetically, the latter from fertilized eggs. Polyploidy is exceptional. In the ichneumonoid wasp Habrobracon juglandis (Ashmead), it has been shown (WHITING 1943) that some diploid males are regularly produced follow-ing inbreeding. Sex is determined by a series of multiple complementary alleles, and the diploid males are sex homozygotes. Haploids are male, and the compounds containing any two of the different sex alleles are female. If a female mates with a male having a sex allele different from her own-a three-allele cross (x a / z b x xc)-all fertilized eggs are female producing. Figure 1 diagrams a two-allele cross in Habrobracon. A diploid female, xa/xb, with 20 chromosomes mates with a haploid male, xa, with ten. Only half of the fertilized eggs are female producing; the other half, the sex homozygotes, xa/xa, are male producing. However, viability of the diploid biparental males is low as compared with their sisters, and these males are near sterile. Their sperm are diploid and their few daughters triploid. There is no chromosome reduction in spermatogenesis of diploid males which appears to be similar in type to that of haploid males. The investigations of INABA (1939) in Japan indicate some interesting raciaI differences between her " Habrobracon pectinophorae (Watanabe) " and the American or European H. juglandis, but the two are not specifically distinct (WHITING 1949). Cytological and genetic studies of H. breuicornis (Wesmael) (SPEICHER and SPEICHER 1940) indicate that this species is similar to juglandis in chromosomal types. In the honeybee, widely separate taxonomically from Habrobracon, sex is likewise determined by multiple complementary alleles (MACKENSEN 1955), although here sex homozygotes are always inviable. Out-crossing is the rule in natural populations of Habrobracon and the honeybee so that two-allele crosses should be very infrequent with their uneconomical pro-duction of bad eggs and diploid males. Unlike these regular relationships between male diploidy and sex determina-tion existing in the economy of these species, polyploidy in the chalcidoid wasp Mormoniella uitripennis (Walker) has arisen by mutation in laboratory cultures. In normal stock, inbreeding produces no change from the simple scheme of five-chromosome haploid males from unfertilized eggs, and ten-chromosome diploid females from fertilized eggs.