Sperm in Competition

Eventually most females of polyandrous lepidopterans mate again, and sperm compete for fertilization of the female's eggs. The outcome of sperm competition is commonly measured as P2, the proportion of offspring sired by the second of two males to mate with the same female. The pattern of sperm competition can either be one of sperm mixing, with no obvious mating order effects, or a first- or second-male sperm priority, where one of the two males fertilizes the majority of the female's eggs (Parker, 1970; Simmons, 2001). A common pattern of sperm precedence is for the last male to mate with the female to fertilize most of the eggs (in around 45% of insect species studied so far: Simmons and Siva-Jothy, 1998; Simmons, 2001). Some species of Lepidoptera show complete second-male sperm precedence (e.g. Trichoplusia ni, North and Holt, 1968; Spodoptera litura, Etman and Hooper, 1979; see Table 3.1). However, there is a more complex pattern of fertilization success that appears to be common in Lepidoptera. Within a species, some females lay eggs fertilized almost exclusively by the first male and some lay eggs fertilized almost exclusively by the second male, whilst very few females lay eggs of mixed paternity. In other words, paternity commonly shows a bimodal distribution (Table 3.1). This is not due to one of the two matings failing to successfully transfer a spermatophore. For most species, identifying the mechanism that underlies the observed pattern of sperm precedence is not straightforward.

Table 3.1. Summary of P2 studies in butterfly (n = 8) and moth (n = 13) species.

Factors Bimodal

Species P2 influencing P2 distribution Reference

Papilionidae

Papilio dardanus 0.67 Pieridae

Colias eurytheme 1.00

Pieris napi 0.66

Pieris rapae 0.77

Nymphalidae

Bicyclus anynana 0.62

Euphydryas editha 0.72

Limenitis arthemis 0.71

Danaidae

Danaus plexippus 0.67

Arctiidae

Utetheisa ornatrix 0.52

Bombicidae

Bombyx mori 0.34

0.95

Gelechiidae Phthorimaea operculella 0.94

Noctuidae

Heliothis virescens 0.99/ 0.47 0.80 0.80

Yes Yes

Relative male size Yes

Sperm number Yes

Male size

Female

Yes Yes Yes

Male size Yes

Male mating history

Relative Yes spermatophore size

Remating interval

Male age Yes

(sperm number) Female size

Clarke and Sheppard, 1962

Boggs and Watt, 1981 Bissoondath and Wiklund, 1997

Wedell and Cook, 1998

Brakefield et al., 2001 Labine, 1966 Platt and Allen, 2001

Oberhauser et al. (cited in

Simmons, 2001) Solensky, 2003

LaMunyon and Eisner, 1993, 1994

Omura, 1939 Suzuki et al., 1996

Rananavare et al., 1990

Flint and Kressin, 1968

Pair et al., 1977 LaMunyon, 2000, 2001

continued

Table 3.1. continued.

Species

Factors influencing P2

Bimodal distribution Reference

Helicoverpa zea 0.71 Pseudaletia separata 0.83 P. unipunctata Pseudoplusia includens Spodoptera frugiperda S. litura

Trichoplusia ni

Tortricidae Choristoneura fumiferana Laspeyresia pomonella

Yes Yes

2nd mating flushes Yes out stored sperm

Pyralidae

Plodia interpunctella 0.68 0.70

Relative Yes spermatophore size and sperm number

Remating interval Yes

Carpenter, 1992 He et a/., 1995 Svard and McNeil, 1994 Mason and Pashley, 1991

Boorman and Parker, 1976 Snow et al., 1970 Etman and Hooper, 1979

Seth et al., 2002a North and Holt, 1968

Brower, 1978 Cook et al., 1997

Retnakaran, 1974 Proverbs and Newton, 1962

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