Pattern of sperm storage

During copulation the male constructs the spermatophore within the bursa copulatrix. Sperm transfer usually occurs towards the end of copulation after spermatophore construction (Callahan and Cascio, 1963). Some time after copulation, sperm leave the bursa copulatrix via the ductus seminalis and are transported, probably aided by female reproductive tract movements (Tschudi-Rein and Benz, 1990; LaMunyon and Eisner, 1993), to the spermatheca (Fig. 3.3). This means that there is little scope for males to directly interfere with sperm already stored from previous matings. Females store sperm in the spermatheca until required for fertilization, which occurs at oviposition. Eupyrene sperm leave the spermatheca through the spermathecal duct, and fertilization occurs when the egg passes the spermathecal duct opening as it travels from the oviduct to the ovipositor (Norris, 1932; Callahan and Cascio, 1963). There is therefore scope for ejaculates of more than one male to be stored in the spermatheca and compete for fertilization. In some species, for example noctuids (Callahan and Cascio, 1963; Holt and North, 1970; Miskimen et al., 1983; He et al., 1995), Manduca sexta (Sphingidae) (Sasaki and Riddiford, 1984) and Pieris napi

(Pieridae) (N. Wedell and PA. Cook, unpublished observations), the spermatheca has two lobes, the utriculus and lagena (Fig. 3.3). In two noctuids, Diatraea saccharalis (Miskimen et al., 1983) and Trichoplusia ni (Holt and North, 1970), it has been observed that the two sperm types are stored in the different lobes: both types move into the utriculus and then apyrene sperm separate and move into the lagena. Sperm numbers in the two parts of the spermatheca are reported for Pseudaletia unipuncta (Noctuidae) (He et al., 1995). These counts show that, although there is not a complete separation of the two sperm types, there is a higher proportion of non-fertile sperm in the lagena than in the utriculus. However, the pattern of separate apyrene storage does not appear to be the rule (Callahan and Cascio, 1963; LaMunyon, 2000).

It is not known whether sperm from several males mix in the spermatheca or are stored in layers with no mixing, so that the last male's ejaculate is predominantly used. If this is the case we expect a drop in last-male sperm precedence and a shift to increasing use of the first (or previous) male's sperm over time, as the last male's sperm is being used for fertilization. However, the few observations we have to date show that P2 does not diminish over time (e.g. Cook et al., 1997). Alternatively, sperm storage may be biased so that females store sperm only from certain 'favoured' males. The pattern of sperm storage will give rise to differences in number of sperm stored with consecutive matings. Sperm mixing and layering predict an increase in number of sperm stored with number of matings, whereas preferential sperm storage necessarily does not. For example, in Spodoptera litura, Etman and Hooper (1979) report that, prior to the sperm from the second male entering the female's spermatheca, sperm from the first male disappears. In the moth Heliothis virescens, despite twice-mated females storing two complements of non-fertile sperm, only one ejaculate's worth of fertile sperm are stored (LaMunyon, 2000). However, this is not the case in all species. In the green-veined white, Pieris napi, for example, twice-mated females store about twice as many apyrene and eupyrene sperm as singly mated females, indicating that ejaculates from more than one male are in direct competition (N. Wedell and PA. Cook, unpublished data).

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