Local mating and sex ratios.

Recently, Hardy and Mayhew1xHardy, I.C.W. and Mayhew, P.J. Trends Ecol. Evol. 1998; 13: 431–432Abstract | Full Text | Full Text PDF | PubMed | Scopus (15)See all References1 discussed the interesting topic of partial local mating in organisms that can facultatively adjust their sex ratios. Although straightforward genetic methods for estimating population structure are useful and have been applied to both fig and parasitoid wasps2xKazmer, D.J. and Luck, R.F. : 107–110See all References, 3xMolbo, D. and Parker, E.D. Jr. Proc. R. Soc. London Ser. B. 1996; 263: 1703–1709Crossref | Scopus (49)See all References, their article reported the work of West and Herre4xWest, S.A. and Herre, E.A. J. Evol. Biol. 1998; 11: 531–548See all References4, who recently inferred fig-wasp population structures using morphological data and applied this to the problem of optimal sex ratios under partial local mating situations. West and Herre4xWest, S.A. and Herre, E.A. J. Evol. Biol. 1998; 11: 531–548See all References4 reasoned that winged-male fig wasps would tend to disperse from their natal patches, and those species with winged males should therefore have sex ratios that are less female biased, as theory generally predicts5xHamilton, W.D. Science. 1967; 156: 477–488Crossref | PubMedSee all References, 6xNunney, L. and Luck, R.F. Theor. Popul. Biol. 1988; 33: 1–30Crossref | PubMed | Scopus (77)See all References. West and Herre4xWest, S.A. and Herre, E.A. J. Evol. Biol. 1998; 11: 531–548See all References4 generally found the predicted sex ratio pattern.As Hardy and Mayhew1xHardy, I.C.W. and Mayhew, P.J. Trends Ecol. Evol. 1998; 13: 431–432Abstract | Full Text | Full Text PDF | PubMed | Scopus (15)See all References1 subsequently point out, another (and possibly the only) study in which wing size was used to infer population structure was a study by King and Skinner7 on the parasitoid wasps Nasonia vitripennis and N. giraulti. In this study, however, the investigators found just the opposite pattern: the male-winged N. giraulti had significantly more female-biased sex ratios than did the male-wingless N. vitripennis7xKing, B.H. and Skinner, S.W. Evolution. 1991; 45: 225–228CrossrefSee all References7. Unsure of the factors influencing the observed sex ratios, they proposed several explanations to account for their data7xKing, B.H. and Skinner, S.W. Evolution. 1991; 45: 225–228CrossrefSee all References7.The reason for this apparent discrepancy is now clear. In a laboratory study using a number of field isolates, Drapeau and Werren8xDrapeau, M.D. and Werren, J.H. Evol. Ecol. Res. 1999; 1: 223–234See all References8 have found that high percentages of N. giraulti females are inseminated before they exit their pupal fly host. Female N. vitripennis rarely, if ever, mate within the host. Therefore, males who disperse from their natal site are unlikely to find virgin females outside of hosts. Hence, the original prediction – that winged species will tend to disperse and find mates – is probably false. Drapeau and Werren qualitatively confirmed King and Skinner's sex ratio data for both species7xKing, B.H. and Skinner, S.W. Evolution. 1991; 45: 225–228CrossrefSee all References, 8xDrapeau, M.D. and Werren, J.H. Evol. Ecol. Res. 1999; 1: 223–234See all References.Interestingly, the males of the third species of Nasonia, N. longicornis, have wings of intermediate size relative to the other two species, and Drapeau and Werren8xDrapeau, M.D. and Werren, J.H. Evol. Ecol. Res. 1999; 1: 223–234See all References8 found that levels of both within-host-mating (WHM) and sex ratios in N. longicornis were intermediate to those of N. vitripennis and N. giraulti. WHM is an exciting behavior because it can rapidly promote barriers to gene flow between sympatric populations.We may briefly contemplate why N. giraulti males have large wings, if male dispersal does not lead to a male fitness advantage. Assuming that WHM does occur in nature (which unpublished field data confirm, see Ref. 8xDrapeau, M.D. and Werren, J.H. Evol. Ecol. Res. 1999; 1: 223–234See all ReferencesRef. 8), WHM might have evolved fairly recently, and male wing-size evolution has therefore lagged behind (note that large male wings appear to be the ancestral state of the Pteromalid wasps). Alternatively, selection might be acting on male wing size either directly (i.e. through courtship cues) or indirectly (i.e. though a pleiotropic effect or linkage disequilibrium), keeping the wing size of male N. giraulti approximately constant. As Hardy and Mayhew1xHardy, I.C.W. and Mayhew, P.J. Trends Ecol. Evol. 1998; 13: 431–432Abstract | Full Text | Full Text PDF | PubMed | Scopus (15)See all References1 point out, cross-species data differences could be due to ‘other differences between a few taxa which also happen to differ in male morphology'. This statement appears to be very relevant to the Nasonia pattern.