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Reproductive plasticity of male responses to larval population density. D-1 Aye Thanda Win

2012/11/27 16:42 に Takashi Matsuo が投稿

Environmental conditions in which the individual is reared have profound effects on its development, juvenile and adult morphology and subsequent reproductive investment [1].

Male resource allocation strategies can be driven by variation in the social environment experienced during larval development. In polyandrous species, armyworm, Pseudaletia separata, males emerged from crowded larvae produced larger spermatophore than males from solitary larvae although body size was not different between the 2 types. Larger spermatophore in crowded-type males may be related to length of female refractory period [3]. Moreover, investment in sperm number or sperm size is also likely to be influenced by environmental conditions during larval development. Sperm number is an important determinant in achieving fertilization success in sperm competition. Males are sensitive to cue future sperm competition risk during their larval development. Consequently, large population densities may reflect increasing risk of sperm competition [4, 5, 7]. In the armyworm, males that emerged from crowded larvae produced significantly more apyrene sperm than those from solitary larvae [4]. Similarly, in the mildly polyandrous species, almond moth, Cadra cautella, high population males transferred more apyrene sperm [5]. Typically, apyrene sperm are associated with reduced female receptivity, suggesting a role in sperm competition. In addition, in scropionfly, adult males resulting from group-bred larvae were larger and produced smaller sperm but transferred at higher rate [7]. It has been suggested that smaller sperm of group-bred might simply be the consequence of increased sperm production (i.e. sperm production predict a trade-off between sperm size and sperm number). Sperm size may be positively associated with sperm competition intensity. Hence, larvae of scropionfly might be sensitive to signals which predict the future risk sperm competition.

Juvenile population size may affect adult potential future mating opportunities and sperm competition. For example, if mates are abundant, males invest more heavily into spermatogenesis in the abdomen (sperm competition). In contrast, if mates are rare, males should invest more into flight musculature in the thorax (mate-searching) [4, 5, 6]. In the moth, Plodia interpunctella, adult males from high densities have relatively larger abdomen and testes, produce greater number of sperm and live for shorter period. At lower densities, however, adult males have relatively larger head and thorax, relatively smaller testes, produce fewer sperm and live for longer periods [4]. Likewise, in the almond moth, low densities males and high densities females survived longer than low densities females [5]. These results indicate that there may be a trade-off between gonadal and mate-searching effort at population level. In contrast, in the dung fly, Scatophaga stercoraria, males reared under high larval density had relatively larger testes and a negative relationship was found between testis size and mate-searching activity but no evidence was found in males reared under low larval density [6]. This result suggests that there is no evidence of population level trade-off between gonadal and mate-searching activity. Thus, plasticity in male reproductive effort in relation to juvenile environment conditions may be widespread among insects.




1.    Peters, T. M. & Barbosa, P. 1977.Influence of population density on size, fecundity and developmental rate of insects in culture. Annu. Rev. Entomol. 22:431-450.

2.    He, Y. & Tsubaki, Y. 1992. Variation in spermatophore size in the armyworm, Pseudaletia separata (Lepidoptera: Noctuidae) in relation to rearing density. Appl. Entomol. Zool. 27:39-45.

3.    He, Y. & Miyata, T. 1997.Variations in sperm number in relation to larval crowding and spermatophore size in the armyworm, Pseudaletia separata. Ecol. Entomol. 22:41-46.

4.    Gage, M. J. G. 1995.Continuous variation in reproductive strategy as an adaptive response to population density in the moth Plodia interpunctella. Proc. R. Soc. B. 261:25-30.

5.    McNamara, K. B., Elgar, M. A. & Jones, T. M. 2009. Adult responses to larval population size in the almond moth, Cadra cautella. Ethol.116:39-46.

6.    Stockley, P. & Seal, N. J. 2001. Plasticity in reproductive effort of male dung flies (Scatophaga stercoraria) as a response to larval density. Funct. Ecol. 15:96-102.

7.    Vermeulen, A., Engels, S., Engqvist, L. & Sauer, K. P. 2009. Phenotypic plasticity in sperm traits in scropionflies (Mecoptera: Panorpidae): Consequences of larval history and seasonality on sperm length and sperm transfer. Eur. J. Entomol. 106:347-352.