Evolution of sex allocation in the genus Tetraneura (Aphididae: Pemphiginae)

Sexuparae (autumnal migrants) of the Pemphiginae produce sexual males and females at once, and the sexuals mature without food intake. Hence, sex ratio and the size of sexuals are completely determined by the mothers (sexuparae). This study examined variation in sex ratio among species of Tetraneura and its allied genera. The relative size of sexuals to sexuparae (mothers) varied greatly among species. In species with relatively small sexuals, each sexupara produced both males and females; the number of males was constant and the number of females tended to increase in larger sexuparae. Sex allocation in these species can be explained by Yamaguchi (1985)’s model. In contrast, in species with relatively large sexuals, a considerable fraction of sexuparae produced sexual females alone. Rearing experiments in T.sp. O showed that sexuparae from one clone produced sexual females only, while those from other clones produced both males and females. In species with relatively large sexuals, the relative investment in both sexes was highly female-biased. Local mate competition has been assumed to be responsible for female-biased sex allocation. However, this comparative study suggests that sex allocation is also affected by the relative size of sexuals. The size of sexual females is highly correlated with that of first instar fundatrices (gall makers). Therefore, it is suggested that advantage of larger size in fundatrices, for example in fighting, has led to larger body size in sexual females and more highly female-biased sex allocation.