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WHO WILL THE FATHER BE? PHILIP DZIUK* Animals normally result from the union of one sperm and one oocyte. Everyone has only one genetic father. Fertilization by more than one sperm—polyspermy—results in nonviability. Too many cooks spoil the soup. When inseminated sperm originate from only one male, competition to fertilize the oocyte exists between sperm within that one male. With the exception of male mice heterozygous for t/t trait, sperm apparently don't have an advantage because of their genetic content [I]. Chimeric males derived from two parental lines can produce sperm from both parental lines and thus can create a situation where sperm from two different strains but in one body compete within a single ejaculate [2]. Not all contests between sperm are within an ejaculate from one male. The situation in which sperm from two males are present at fertilization has been termed "heterospermic insemination," to distinguish it from the conditions in which sperm from one male are excluded as a result of physical, behavioral , or physiological barriers that give an egg an exclusive access to one male [3, 4]. The question that arises with heterospermic insemination is, "Who will the father be?" How is it decided? As a result of many controlled studies, several conclusions can be reached. First there are inherent differences between males [5-15]. Genetic Differences Between Males Influence Paternity This fertility differential between males seems to have a very significant genetic basis. Males of certain strains of mice and chickens are much more likely to sire a very disparate proportion of offspring when equal numbers of sperm are mixed and inseminated [5, 8-10, 15-18]. The fertility of a male or a strain of males relative to another remains quite constant over *Department of Animal Sciences, University of Illinois, Urbana, IL 61801.© 1998 by The University of Chicago. All rights reserved. 0031-5982/98/4103-1061$01.00 Perspectives in Biology and Medicine, 41, 3 ¦ Spring 1998 | 439 a considerable period [17, 18]. If you have it, you usually keep it: if you don't have it, you don't acquire it. Timing ofInsemination Relative to Ovulation The differential fertility between males can be modified by two means. The timing ofan insemination relative to the timing of the alternate insemination will give one male an advantage. The timing of an insemination relative to the time of ovulation will also influence the proportion of offspring sired. It may at first seem that the more closely the time of deposition of sperm coincides with the time of ovulation of the oocyte, the greater the opportunity for optimal fertility. However, sperm requires a period of residence in the female reproductive tract before fertilization can occur. This phenomenon is termed capacitation. The minimum period in the pig is about two hours, whereas in the rat and rabbit it is about six hours. These periods of residence arejust the minimum, whereas the period for optimal fertilizing potential has been found to be about 10 to 12 hours in rabbits, pigs, cattle, and sheep. It is possible that future studies would reveal greater variation between species. Several studies have focused on the importance of the interval from insemination to ovulation on the likelihood of paternity. Early observations on the rabbit revealed that when buck E was mated to a doe at 10 hours before ovulation, followed by a mating to buck C at four hours before ovulation, all offspring were sired by the first buck E [19]. When the reciprocal regimen of buck C first and buck E second was used, buck C sired only 60 percent of offspring. When a third buck A was mated first, followed by buck C, buck A sired only 55 percent of the offspring . Buck C sired 97 percent of the offspring when it mated first and buck A was second. These experiments suggest that the innate advantage certain males have is accentuated when time of insemination is modified, in spite of both matings occurring well before ovulation. A less fertile male can partially overcome his disadvantages by timeliness. When the interval between the first and second matings was reduced to 30 to 60 minutes in a subsequent study...


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