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UBC Theses and Dissertations

An analysis of intraovarian sperm interaction in the guppy, Poecilia reticulata Grove, Bryon Dennis

Abstract

Although guppy females may store sperm in the ovary for several months, they become sexually receptive for a short time every 22-30 days. Females may remate during these receptive periods and consequently, a potential competition between stored and fresh sperm for oocytes may occur. Earlier studies indicated that sperm from the most recent insemination dominates fertilization of new broods. However, these studies provide no clues to how this apparent sperm precedence occurs. There are three possible explanations. First, sperm from new inseminations may outnumber stored sperm and thus, have a statistical advantage. Second, since fertilization in guppies occurs within a few days of insemination, this timing of insemination (relative to egg maturity) may give fresh sperm an advantage. Finally, the location of fresh sperm in the ovary may give them greater access to mature eggs. In this study, females were artificially inseminated with known numbers of spermatophores to see how the proportion of one male's sperm in a mixed insemination determines its contribution to new broods; 2.) the number of spermatophores in inseminations affects the degree of precedence shown by sperm from re-inseminations; and 3.) the timing of re-insemination relative to the female reproductive cycle affects the degree of precedence shown by re-inseminations. An autosomal, recessive allele was used as a genetic marker to distinguish between different males' sperm. In addition, ovaries of females artificially inseminated twice were examined histologically to locate sperm from each insemination. Sperm from one of the inseminations was radioactively labelled. These experiments indicated that sperm precedence does not necessarily result from a statistical advantage to sperm from a re-insemination. Also sperm from re-inseminations occurring up to a week before a new batch of oocytes is mature have strong precedence over stored sperm, although the degree of precedence may be reduced. Autoradiographical analysis of sections of ovaries, however, indicated that the location of sperm storage sites may contribute to sperm precedence. Sperm clusters found in tubules closely associated with oocytes come predominantly from re-inseminations. A model is proposed suggesting that movement of sperm into these tubules immediately after insemination accounts for the dominance of new inseminations during fertilization. The possible functional significance of sperm storage and sperm precedence in poeciliids is discussed.

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