A Possible Method of Sex Ratio Determination in the Parasitic Hymenopteran Nasonia vitripennis

Abstract

Nasonia vitripennis (Walter) (Pteromalidae) is parasitic on the pupal stage of museoid flies. The female drills through the host puparium and lays eggs on the pupa within. When a population of N. vitripennis has ready access to host puparia, each generation of the offspring is composed of females and males in the ratio 3: 1¹. The females are diploid and the males haploid². So far, no suggestion has been made as to how this sex ratio may be determined; but it seems possible that it may be affected by the arrangement of the eggs in the ovariole before fertilization. The eggs are shaped like curved sausages and are more pointed at one end than the other. This is the typical hymenopterous shape. Owing to its curvature, each egg has in its meridional plane a convex and concave side. Within each ovariole the eggs are orientated so that the oldest eggs, namely those nearest the oviduct end of each of the four ovarioles which comprise a single ovary, almost always have their convex aspect pointing away from the axis of the ovary as a whole (Fig. 1). This arrangement was observed in 46 out of 50 insects examined.