Fertilization in Drosophila: I. Evidence for the regular occurrence of monospermy

Abstract

Contrary to the widespread belief that polyspermy is a normal characteristic of fertilization in Drosophila, we obtained evidence indicating that, as a rule, only one sperm is present per fertilized egg in two species investigated. Feulgen-stained whole mounts of D. melanogaster and D. virilis meiotic eggs, and serially sectioned eggs of D. melanogaster stained with Feulgen or with azur-eosin-Giemsa were examined. No polyspermy was observed among 87 fertilized eggs of D. virilis; among 165 fertilized eggs of D. melanogaster, only 6 were found to be dispermic. An autoradiographic series in which we combined tritiated thymidine labeling and Feulgen staining of sperm fully supported the validity of the Feulgen technique as a means of determining the degree of polyspermy in Drosophila. In this series each sperm was found to be Feulgen positive and also labeled. Nineteen of 20 eggs examined were monospermic, one was dispermic. A survey of the literature dealing with insect fertilization has led us to conclude that in a large number of insect species (1) physiological polyspermy does not occur and (2) when accessory sperm do occasionally enter the egg, pathological effects of polyspermy need not be manifested. The reaction to polyspermy of these species should be considered as separate from Type I Inhibition (found in those eggs in which polyspermy is pathological) and from Type II Inhibition (found in those eggs in which polyspermy is physiological). We propose to name this reaction Type III Inhibition, with the stipulation that it does not involve a new kind of inhibitory process against polyspermy, but simply combines features of the other two types of inhibition.