Mosaicism in Habrobracon juglandis associated with the ebony locus.

MONG species of insects that have been used for genetic studies, there Aoccasionally appear adults which are composed of two or more different genomes. In Drosophila melanogaster, where most of these mosaics were of the gynandromorph type, it has been shown that elimination of an X chromosome during early cleavage was involved (MORGAN and BRIDGES 1919; PATTERSON 1931). In a few instances, fertilization involving two egg pronuclei and two sperm was suggested (BRIDGES 1925). In D. pseudoobscura, CREW and LAMY (1938) have obtained unisexual mosaics of the haplo-diploid type and have suggested that the sperm pronucleus divides before union with the egg pronucleus, that one of these cleavage nuclei unites with the egg pronucleus to form a diploid nucleus while the other sperm nucleus continues to divide and to give rise to haploid tissue. Among insect species with haploid parthenogenesis, the gynandromorph types contain diploid female structures arising from the union of an egg and a sperm pronucleus, and haploid male structures arising from an accessory pronucleus. In Habrobracon juglandis the accessory pronuclei may be of maternal or paternal origin (P. W. WHITING 1932; A. R. WHITING 1961). In the honey bee, Apis mellifera, dispermy is involved ( ROTHENBUHLER, GOWEN and PARKS 1952). In Habrobracon, haploid mosaic males derived from unmated heterozygous females occur and have been shown to consist of genetic material from two and sometimes three genetically different pronuclei (A. R. WHITING 1961; VON BORSTEL 1957). In haploid organisms, a model based on chromosome elimination does not apply since such loss would be lethal. The fact that mosaicism appears more frequently in some stocks than in others and sometimes in association with a single mutant gene may indicate that the production of mosaics is under genetic control. There may be a relation between specific types of mosaics and specific mutants, and thus the particular developmental events that precede the union of egg and sperm nuclei may each be under control of a gene. If this is true, then the process of fertilization can be studied from the standpoint of developmental genetics. The present paper shows that events leading to fertilization are under genetic control. This conclusion is based on a study of mosaics in Habrobracon jugandis that are associated with the ebony locus.