Sperm Competition

When the probability of female remating is high, selection should favor adaptations in males that allow them to reduce or avoid competition with the sperm of another male. This can lead to rapid and divergent evolution of traits that function in sperm competition and its avoidance. These traits may be manifest in behavior (e.g., mate guarding), genital morphology (e.g., structures that deliver sperm closer to the spermatheca), and physiology (e.g., chemicals in the ejaculate that enhance the success of sperm). Selection may also act at the level of the sperm itself, in that some may be adapted to outcompete others for access to eggs (Ridley, 1988; Eberhard, 1996; Simmons, 2001).

In studies of sperm competition paternity is typically reported as P2, the proportion of offspring sired by the last male to mate with a female in controlled double mating studies (Parker, 1970). A P2 between 0.4 and 0.7 indicates sperm mixing. A P2 higher than 0.8 suggests that sperm are either lost prior to the second mating, or that second-male sperm precedence or displacement is in operation. Values of < 0.4, where the first male is favored, are rare (Simmons, 2001).

Classical studies of sperm competition have been conducted in two cockroach species: B. germanica and D. punctata. Cochran (1979b) studied the phenomenon in the German cockroach and used the genetic mutant rose eye to recognize paternity. In the single instance of a female mating twice prior to the first egg case, the second male sired 95% of the eggs. Just over 20% of females re-mated between egg cases; Gwynne (1984), using Cochran's data, calculated the P2 of these to be 0.43. Using a slightly different approach with the same data, Simmons (2001, Table 2.1) calculated the P2 as 0.69 when mutant males were the first to mate and 0.33 when wild-type males were the first to mate. The P2 calculated using mixed broods only was < 0.37 (Simmons, 2001, Table 2.3). Blattella is exceptional, then, in that the general

Fig. 6.6 Sperm competition in Blattella germanica. Virgin females with the recessive eye color mutation rose eye were initially mated to a mutant male, then to a wild-type male (top graphs), or first to a wild-type male, and subsequently to another mutant (bottom graphs). In each case the female was exposed to the second male only after her first egg case began protruding; progeny of the first egg case were thus sired exclusively by the initial male. Inset graphs detail the paternity of nymphs from oothecae of mixed parentage, that is, those containing eggs fertilized by both males. After data in Cochran (1979b), with permission of D.G. Cochran.

Fig. 6.6 Sperm competition in Blattella germanica. Virgin females with the recessive eye color mutation rose eye were initially mated to a mutant male, then to a wild-type male (top graphs), or first to a wild-type male, and subsequently to another mutant (bottom graphs). In each case the female was exposed to the second male only after her first egg case began protruding; progeny of the first egg case were thus sired exclusively by the initial male. Inset graphs detail the paternity of nymphs from oothecae of mixed parentage, that is, those containing eggs fertilized by both males. After data in Cochran (1979b), with permission of D.G. Cochran.

trend is first-male precedence. A focus on average P2 values can be misleading, however, because variation within a species can be extreme (Lewis and Austad, 1990; Eberhard, 1996). A detailed examination of Cochran's data indicates that in most reproductive episodes, the eggs of some oothecae were exclusively fathered by the first male, some were exclusively fathered by the second male, and some were of mixed parentage (Fig. 6.6). In the waning stages of the female's reproductive life sperm from the second male sired a higher proportion of the offspring, suggesting that remating may occur in response to declining sperm supply (Cochran, 1979b). Maternal influence may account for some variation in paternity. Females have four spermathecae, each with a separate opening, and thus potential for selective sequestration and release of sperm (discussed below). It is noteworthy, based on the P2 values cited above, that either the sperm of mutant males are somewhat inferior competitors, or that females exhibit some preference for the sperm of wild-type males.

Woodhead (1985) used irradiated males to examine sperm competition in D. punctata, a viviparous cockroach that remates only after partition of the first brood. The P2 averaged 0.67 but was higher when the second male was the normal male (0.89), rather than the irradiated male (0.46). Plots of the position of viable versus sterile eggs in individual oothecae suggested sperm mixing; there was no consistent spatial pattern of egg fertilization by the two sires. The spermatheca in Diploptera females is tubular, a shape usually associated with sperm stratification (Walker, 1980).

Variation in Ejaculates

A number of studies indicate that males increase the size of their ejaculate in the presence of rival males (summarized in Wedell et al., 2002). Harris and Moore (2004) tested the idea in N. cinerea by exposing adult males during their post-emergence maturation period to the chemical presence of potential competitors (other males) or mates (females); spermatophore size, testes size, and sperm numbers were then determined and compared to isolated male controls. The authors could not demonstrate an influence of male competitors on testes size or sperm number. Spermatophore size increased in the presence of either sex, suggesting the possibility of a group effect on this reproductive character. Males did transfer significantly more sperm during copulation when, after adult emergence, they matured in the presence of females rather than males. One caution in interpreting this study is that the development of the testes and the production of sperm in Nauphoeta may be largely complete prior to adult emergence, as it is in G. portentosa, Byrsotria fu-migata (Lusis et al., 1970), Blatta orientalis (Snodgrass, 1937), and P. americana (Jaiswal and Naidu, 1972).

Hunter and Birkhead (2002) addressed the relationship between sperm competition and sperm quality by comparing the viability of male gametes in species pairs with contrasting mating systems. They found a higher percentage of dead sperm in N. cinerea, which the authors considered monandrous, than in D. punctata, which they considered polyandrous. It is unclear, however, as to how much the mating systems in these two species differ. Female Diploptera typically mate just after adult emergence, then carry the spermatophore until shortly before the ootheca is formed. They readily remate after partition of the first brood (Stay and Roth, 1958; Woodhead, 1985). Similarly, virgin female Nauphoeta are unreceptive after their first copulation; after partition, they may or may not mate again (Roth, 1962). Females of both species, then, may be considered monandrous during their first reproductive period, but polyandrous over the course of their lifetime.

Was this article helpful?

0 0

Post a comment