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Last modified
7/14/2009 5:01:48 PM
Creation date
5/20/2009 11:07:23 AM
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UCREFRP
UCREFRP Catalog Number
9643
Author
Seener, J. W.
Title
Inbreeding Depression and the Survival of Zoo Populations
USFW Year
n.d.
USFW - Doc Type
209-217
Copyright Material
YES
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<br />Sex Ratio Depression <br /> <br />Sex ratio also changes during inbreeding, apparently because the male <br />X chromosome (in mammals) is always hemizygous, independent of in. <br />breeding. A female X chromosome pair can qeoome increasingly homozy. <br />gous with an increase in inbreeding. Therefore, males become increasingly <br />common among the survivors at higher levels of inbreeding. This shift in <br />sex ratio can be seen in Table 2 in Chapter 9. A more precise study of the <br />sex ratio change resulting from sex chromosome inbreeding is given by <br />Hook and Schull (1973). The data in this paper and that in Table 2 of <br />Chapter 9 result in estimates for the value of Ba (obtained in a similar <br />fashion to B1) of about 1.5, although 0.5 is used as the standard value in <br />all other figures. An uneven sex ratio affects population survival in two <br />ways: directly, by lowering the probability of at least one individual of <br />each sex surviving to reproduce and indirectly, by increasing the rate of <br />inbreeding. Nevertheless, Figure 5 shows that the intensity of sex ratio <br />depression is not sufficient to greatly affect the probability of population <br />survival. <br />If inbreeding depression results from the mutation of "good" genes to <br />"bad" genes, which are lethal or deforming when in the homozygous con- <br />dition (mutational load), it should be possible to "cleanse" the animal of <br />its deleterious mutations by inbreeding, then culling defects and mating <br />survivors to restore th~animal's natural variability and evolutionary p0- <br />tential. On the other hand, some loci confer greater fitness when hetero- <br />zygous (segregational load). At these loci inbreeding increases the <br />proportion of the genome which is homozygous and inevitably decreases <br />mean fitness. Sickle-cell anemia, which occurs in some human popula. <br />tions exposed to malaria, is an example. Inbreeding could 'icleanse" a <br />population of the sickle-cell defect, but would also result in lowerresis- <br />tance to malaria. Fitness of the population as a whole is higher with tha <br />sickle-cell gene than without it when malaria is present. <br />While there is considerable controversy on the relative contribution of <br />the two types of genetic load to inbreeding depression, there is little <br />doubt that at, least some load is due to the second (balance) cause <br />(Lewontin, 1974). For that reason, it would be imprudent to change the <br />level of inbreeding depression in a captive population by attempting to <br />purge the population of its load (Chapter 9). <br /> <br />Genetics of the Founders <br /> <br />What can be done to enhance the survival of a species that has been <br />picked for a captive breeding program? If the relationship between ge- <br />netic variability and productivity found in most studies of inbreeding de- <br />pression is generally troe, two rules immediately follow: <br />1. Avoid starting a population with animals which are already inbred. <br /> <br />214 <br /> <br /> <br />SENNER/CHAPTER12 <br />INBREEDING DEPRESSION AND THE SURVIVAL OF ZOO POPULATIONS <br /> <br />100 <br /> <br />100 <br /> <br />Cl <br />z <br />:> <br />:> <br />a: <br />::l <br />en <br />I- <br />z <br />w <br /><..> <br />a: <br />w <br />0.. <br /> <br /> <br />o <br /> <br />o <br /> <br />FIGURE 6. The senaitlvity of the <br />model to the inbreeding coefficient <br />of founder animals. The lower <br />curve reflects the efFect of starting <br />with founders which are <br />completely inbred but unrelated. <br /> <br />A group of unrelated but completely inbred animals has ~lf as ~u~h <br />variability as unrelated animals that are not inbred. The ~e:ence IS il- <br />lustrated in Figure 6. The standard case assumes that the lmttal ~pula- <br />tion is slightly inbred so that average f is 0.05 as might be found In rare <br />and declining species. . . <br />2. Do not start a population with related animals. Figure 7 illustrates <br />the effect population relatedness has on survival. The extreme ~alue of <br />0.25 represents the case where the founder group is composed of Sibs. The <br />standard case assumes that relatedness of the initial population is 0.05, ~ <br />might be found in a rare species. One need not look far ~ .find cases In <br />which these rules are violated, usually because of the realities of 1:?e ac- <br />quisition process. A serious captive breeding program shoul~ lDVest <br />enough care in the acquisition phaSe to assure that 88 many ammals as <br />Possible are as unrelated as possible. <br /> <br />FIGURE 6. The sensitivity of the <br />model to changes in the level of <br />sex ratio inbreeding depression. <br /> <br />Initial Population Size <br /> <br />Two population size parameters have an important effect on s~va1: <br />the initial effective population size (1) and the maintenance effective pop- <br /> <br />216 <br />
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