The evolution and function of the spermatophylax in bushcrickets (Orthoptera: Tettigoniidae)
In certain species of cricket and bushcricket (Orthoptera; Ensifera), the male transfers an elaborate spermatophore to the female at mating. This consists of a sperm-containing ampulla and an often substantial, sperm-free, gelatinous mass known as the spermatophylax. After mating, the female eats the spermatophylax before consuming the ampulla. The spermatophylax is particularly well developed in the bushcrickets (Tettigoniidae) and can contribute to a loss of as much as 40% of male body weigh at mating in some species. Recently, there has been considerable debate over the selective pressures responsible for the evolution and maintenance of the spermatophylax and other forms of nuptial feeding in insects. Two different, though not mutually exclusive, functions have been suggested for the spermatophylax: 1) nutrients from the spermatophylax may function to increase the weight and\or number of eggs laid by the female, i.e. may function as paternal investment; 2) the spermatophylax may function to prevent the female from eating the ampulla before complete ejaculate transfer, i.e. may be regarded as a form of mating effort. In this study, a comparative approach combined with laboratory manipulations were used in an attempt to elucidate the selective pressures responsible for the origin, evolutionary enlargement and maintenance (= function) of the spermatophylax in bushcrickets. The results suggest that the spermatophylax originated as an adaptation to maximise ejaculate transfer by countering the tendency of females to eat the ampulla prematurely. The spermatophylax appears to be analogous to a range of adaptations found in males of the sub-order Ensifera, which may be interpreted as functioning to maximise ejaculate transfer. These adaptations include prolonged copulation following spermatophore transfer, feeding the female with glandular secretions following spermatophore transfer, post-copulatory mate guarding and multiple copulations with the same female. The occurrence of prolonged copulation following spermatophore transfer appears to be associated with the total loss of the spermatophylax in the meconematine bushcricket Meconema and with the considerable reduction in spermatophylax size in the ephippigerine bushcricket Uromenus rugiscollis. This supports the hypothesis that prolonged copulation and the spermatophylax are analogous in function. The subsequent evolutionary enlargement of the spermatophylax appears to have accompanied the evolutionary enlargement of ejaculate volume and sperm number, i.e. appears to have proceeded to facilitate the transfer of larger ejaculates. A comparative study of 43 species of bushcricket revealed a positive relationship, across taxa, between evolutionary changes in spermatophylax size and changes in ampulla size (i.e. ejaculate volume) and sperm number, with male body weight controlled for. The current function of the large spermatophylax appears to be the same as that of the small spermatophylax, i.e. to ensure complete sperm \ ejaculate transfer. No significant difference in the shape of the sperm transfer curve relative to the mean duration of spermatophylax consumption was found between Leptophyes punctatissima (small spermatophylax) and L.laticauda (large spermatophylax). Furthermore, in L.laticauda, males appear to adjust the size of the spermatophylax in relation to the amount of sperm or volume of ejaculate they are able to produce: a positive relationship was found between spermatophylax mass and sperm number and between spermatophylax mass and ampulla mass (i.e. ejaculate volume). The possibility that the spermatophylax additionally functions as paternal investment cannot, however, be ruled out on this basis. In order for male-donated nutrients to function as paternal investment they must 1) have a positive effect on offspring fitness and\or number and 2) the nutrient donating male must stand to fertilise most or all of the offspring which benefit from his nutrients. A positive effect of spermatophylax consumption on egg weight and\or number has previously been documented in some species of bushcricket, though has not been found in others. In this study, no effect of spermatophylax consumption on female reproductive output was found in L.punctatissima, L.laticauda, or Steropleurus, even when, in the latter two cases, females were maintained on a restricted diet. Furthermore, in L.punctatissima and Steropleurus stali (though not in L.laticauda) it appears that the spermatophylax-donating male is unlikely to fertilise eggs in which his nutrients might be incorporated, in light of the short female re-mating interval, the pattern of last-male sperm precedence and the pattern of oviposition. The enormous spermatophylax of S.stali is unlikely, therefore, to function as paternal investment. Recent studies suggest that in a number of other bushcricket species, including some with very large spermatophylaxes, the spermatophylax is also unlikely to function as paternal investment for the above reasons. In conclusion, while the paternal investment hypothesis lacks generality, the ejaculate-protection hypothesis seems to be more widely applicable and appears to successfully account for the origin, evolutionary enlargement and current function of the spermatophylax in bushcrickets.