| Summary: | The mode of sex determination in most Hymenoptera is that of arrhenotokous parthenogenesis, i.e. females originate from fertilised eggs and males develop parthenogenetically from unfertilised ones. Females can therefore determine the sex of each of their offspring by controlling fertilisation. This has led to much speculation about the factors that govern hymenopteran sex ratios. Many sex ratio theories predict that solitary Hymenoptera should produce male and female offspring in a ratio of 1:1. The limited data available on sex ratios in solitary Hymenoptera does not support this prediction. However, for various reasons, data may not be representative and few detailed studies have been done. The purpose of this study was, therefore, to examine sex ratios of Coccophagus atratus, a solitary hymenopteran parasitoid, to determine whether sex ratios in this species approach 1: 1. C. atratus is a suitable species for sex ratio studies because male and female eggs are deposited in different hosts. The sex of deposited eggs can, therefore, be determined at oviposition. This eliminates any possible effects of differential mortality. Laboratory experiments were designed to determine whether C. atratus females deposit male and female eggs in fixed proportions or in a set sequence. In addition, samples of hosts were collected from the field to assess C. atratus sex ratios under natural conditions. Results show that C. atratus females do not produce sex ratios that approach 1:1 and male and female eggs are not deposited in a set sequence. Sex ratios are variable and appear to be related to the relative abundance of hosts suitable for female development and for male development. These results are discussed in relation to current sex ratio theories.
|
|---|
