Genetic Segregation in the Unisexual Wasp Devorgilla.

T H A T genetic mutations are prerequisite to evolution of the species is well recognized. The mechanism by which such mutations are segregated and recombined in bisexual species forms a large part of classical genetics. Less well known are the modifications, in unisexual species, of the same mechanisms to attain comparable ends. The segregation of genes in aberrant cases of diploid parthenogenesis has been described for Habrobracon (K. G. SPEICHER 1934), Drosophila (STALKER 1954; CARSON 1962), for the axolotl (LINDSLEY, FANKHAUSER, and HUMPHREY 1956) and for the honey bee (TUCKER 1958). In each of these cases normal reproduction is biparental. In this paper we analyze segregation in a species of wasp where males are virtually unknown. Deuorgilla canescens (Grav.) , known also in the literature as Nemeritis canescens (Grav.), and more recently under the generic names Idecthis and Exidecthis ( MUESEBECK, ROMBEIN and TOWNES 195 1 ) , is a cosmopolitan ichneumonid endoparasite of the Mediterranean flour moth, Anagasta kiihniella (Zeller) , and its relatives. Parasitization is accomplished by injection of an egg into the body of the host caterpillar, which remains active until pupation. Although many eggs may be laid individually in a single host (over 100 have been recovered), invariably only a single adult wasp emerges, a result of larval cannibalism. Over a decade of breeding this species in our laboratory, and a considerably longer time spent by PROFESSOR S. E. FLANDERS at the Citrus Research Center, Riverside, California (FLANDERS and BADGLEY 1963), have failed to uncover a single functional male. One male was produced during the present experiment, from an X-rayed mother, but it was cytologically diploid and functionally sterile. Reproduction, then, is essentially, if not totally, thelytokous, at least in the strains studied. The authors are grateful to DR. FLANDERS for supplying the strain on which the present work was done. Oogenesis in this species (described by B. R. SPEICHER 1937) should be understood for the purposes of this paper. Adult females have 22 chromosomes in diploid oogonial cells. The diploid number is retained in the nucleus of the matured, unfertilized egg by virtue of an abortive first maturation division. I t is ineffective because the dyads, which would ordinarily remain at the periphery of the egg to form the first polar nucleus, return to the vicinity of the second