Evidence of capacitation in the parasitoid wasp,Nasonia vitripennisand its potential role in sex allocation

Abstract

The allocation of resources to the production of one sex or another has been observed in a large variety of animals. Its theoretical basis allows accurate predictions of offspring sex ratios in many species, but the mechanisms by which sex allocation is controlled are poorly understood. Using previously published data we investigated if alternative splicing, combined with differential expression, were involved with sex allocation in the parasitoid wasp, Nasonia vitripennis. We found that sex allocation is not controlled by alternative splicing but changes in gene expression, that were identified to be involved with oviposition, were shown to be similar to those involved in sperm motility, and capacitation. Genes involved in Cholesterol efflux, a key component of capacitation, along with calcium transport, trypsin and MAPKinase activity were regulated in ovipositing wasps. The results show evidence for regulation of sperm motility and of capacitation in an insect which, in the context of the physiology of the N. vitripennis spermatheca, could be important for sex allocation.Understanding the molecular mechanisms controlling an organisms response to their en-2 vironment, is one of the key questions of biology. A fundamental response to the environ-3 ment is altering the ratio of male and female offspring, this is, sex allocation (Charnov, 4 1982; West, 2009). Sex allocation has a large body of theoretical work, and supporting ex-5 perimental evidence describing its evolutionary role in many taxa (West, 2009). However, 6 outside of circumstances such as temperature dependent sex determination (Göth Ann 7 and Booth David T, 2005; Bull and Vogt, 1979), there is little known about the molec-8 ular controls of sex allocation. Frequency dependent selection, was proposed by Fisher 9 to explain sex allocation dynamics (Fisher, 1999). However, when Hamilton derived kin 10 selection, he realised that population level competition in species with limited dispersal, 11 would result in competition between kin. He proposed that selection would act to optimise 12 species sex allocation, in order to minimise competition between kin (Hamilton, 1967). 13This is Local Mate Competition (LMC) and it provides accurate empirical estimates of 14 optimal sex ratios, based upon population structure. It's predictions have been supported 15 by observations in mammals, fish and in many invertebrates (West, 2009; Charnov, 1982). 16LMC is particularly well studied in the parasitoid wasp, Nasonia vitripennis, a model for 17 the study of sex allocation. 18Nasonia vitripennis produces more female biased broods under conditions of high 19 LMC, both in the wild and in laboratory conditions (Werren, 1980(Werren, , 1983b. Several 20 factors have been found to alter their sex allocation, including the host and brood size 21 (Werren, 1983a; West, 2009). The two main cues females use to alter sex allocation are; 22 1) if the host has been previously parasitised and 2) the number of local conspecifics 23 (Werren, 1980(Werren, , 1983a Shuker et al., 20...