Sexual Isolation in a Parthenogenetic Strain of Drosophila mercatorum

(1) Studies were made to determine if any natural sexual isolation exists between a parthenogenetic strain and two bisexual strains of D. mercatorura and if sexual isolation may be enhanced by selection in the parthenogenetic strain. (2) In experiments where equal numbers of parthenogenetic and bisexual strain females were used, the parthenogenetic strain showed significant sexual isolation. Neither of the two bisexual strains, furthermore, exhibited significant sexual isolation when equal numbers of each were used in similar tests. (3) Natural sexual isolation has thus arisen in the parthenogenetic strain in the absence of any selection for isolation. It apparently occurred as the result of random drift in the small laboratory population involved. (4) In experiments where equal numbers of the original parthenogenetic strain and parthenogenetic substrain females, which had been selected for sexual isolation, were used, one of the substrains showed a sexual isolation significantly greater than the parent strain. However, one of the selected parthenogenetic substrains did not exhibit enhanced sexual isolation. (5) Sexual isolation in the selected parthenogenetic substrain probably has a simple genetic basis involving a few gene loci rather than an extensive polygenic basis. (6) The present study gives strength to the views of Carson and Stalker that a wholly parthenogenetic strain of flies may develop from one with a low rate of parthenogenesis when a male shortage or some form of sexual isolation exists.