An assessment of the threat to Grevy's zebra from hybridization

Hybridization between two closely related species is a natural evolutionary process that can lead to the divergence of two species through reinforcement, the absorption of one species by another through persistent genetic introgression, or the formation of new species (Roy et al., 1996; Arnold et al., 1999; Lowe, Harris & Ashton, 2004). For evolutionary biologists, therefore, hybridization represents a phenomenon with compelling implications, because of its potential for increasing biological and genetic diversity over evolutionary timescales (Seehausen, 2004). For conservationists on the other hand, hybridization is almost always seen as a threat to biodiversity because small populations of threatened or endangered species are likely to be lost if they hybridize extensively with more abundant congeners (Levin, Francisco-Ortega & Jansen, 1996; Allendorf et al., 2001; Pasachnik et al., 2009). Cordingley et al. (2009) report just !such a case of hybridization between the endangered Grevy's zebra and the more abundant plains zebra. The concern here is whether the Grevy's zebra is at risk of further endangerment as a result of this newly detected hybridization. The specific concern in the case of Grevy's zebra is genetic introgression, and consequently genetic swamping of the rare species, Grevy's, by the abundant one, plains zebras. Plains zebras are less at risk from this genetic introgression, because the large genetic reservoir available in the plains zebra population will dilute the impact of hybridization on plains zebras, whereas rare species such as Grevy's do not have this buffering effect of a large population (see Hailer & Leonard, 2008). The rate of hybridization appears to be low in this case, with a limited number of hybrid individuals detected in only a small part of the range of overlap between the two species (Cordingley et al., 2009). The low rate of hybridization, however, is not sufficient to dismiss conservation concerns, because the Grevy's population in question is very small (<20 animals, compared with 4000 plains zebras in the same area). The direction of the introgression is also relevant to assessing the threat. Specifically, genetic swamping of a rare species by a common species may be accelerated if hybridization is caused by males of an abundant species forming hybrids with females of the rare species; if rare females are disproportionately ‘tied up’ by hybrid matings, population growth of the rare species may slow dramatically. For example, the population of the threatened Canadian lynx is being compromised by apparently unidirectional hybridization between female Canadian lynx and male bobcats (Schwartz et al., 2004). Interestingly, the Grevy's zebras reported by Cordingley and colleagues show unidirectional hybridization in the other direction. Grevy's zebras are larger than plains zebras, and Grevy's males are evidently able to gain access to plains females more readily than the reverse; Cordingley and colleagues found no evidence of hybridization between plains males and Grevy's females, and a clear evidence (through analysis of mtDNA and Y-chromosome markers) of hybridization between Grevy's males and plains females. Further, because the mothers of hybrids are invariably plains females, it appears that hybrids are socialized as plains zebras, and become integrated into plains zebra society; the Grevy's population itself remains free of hybrid individuals. Consequently, Cordingley and colleagues conclude that hybridization represents no immediate threat to the Grevy's. On the other hand, Cordingley and colleagues also report the observation of two Grevy's females with plains zebra mtDNA. They explain this occurrence as probably due to misidentification of the zebra's phenotypes (i.e. they propose that the sampled individuals were in fact plains zebras mistaken as Grevy's, because those particular samples were taken in dense vegetation). However, the observation of Grevy's females with plains mtDNA would be entirely consistent with the unidirectional hybridization they have documented; even though F1 hybrids integrate into plains zebra social groups, the directionality of hybridization (Grevy's males mating with plains zebra females) could easily result in backcrossing of these F1 females with Grevy's males, and even subsequent backcrossing of the F2 hybrids, producing increasingly Grevy's-like females with mtDNA from the plains zebra. This process will require no special environmental or population conditions, and without an analysis of biparentally inherited markers that are unique to each species, it is difficult to dismiss Cordingley and colleagues observation as a sample collection error. Indeed, this pattern of introgression is quite common in species of conservation concern. For instance, many American bison populations have extensive introgression of mtDNA from domestic cattle (Ward et al., 2001). Other examples include unidirectional introgression of coyote mtDNA in sympatric and hybridizing populations of threatened gray wolf populations (Lehman et al., 1991), unidirectional introgression of baboon mtDNA into at least one population of the recently discovered kipunji monkey (Roberts et al., 2009; Zinner, Arnold & Roos, 2009), and unidirectional introgression of forest elephant mtDNA into some populations of savannah elephants, probably during the last ice age (Roca, Georgiadis & O'Brien, 2005). In many of these cases, the observed pattern of introgression would not be predicted from known dispersal patterns and social behavior. For instance, how did female offspring of female baboon progenitors, which are strongly philopatric, end up living in kipunji social groups? Yet such introgression has incontrovertibly occurred in a number of cases. A similar unidirectional genetic introgression of plains zebra mtDNA into the Grevy's zebra is possible in the hybridization study by Cordingley and colleagues. The hybridization reported by Cordingley and colleagues is recent, and is ultimately traceable to anthropogenic factors; habitat loss, leading to small Grevy's populations and, in the case they report, an apparently stochastic fluctuation toward males in the sex ratio of their study population of Grevy's zebras. They correctly conclude that the most immediate threat to the Grevy's zebra is declining population size caused by habitat loss, and that this requires much more urgent attention than hybridization, which they appropriately treat as a symptom of a declining population. Removal of hybrids would do little or nothing to help conserve the Grevy's population, and would consume valuable resources. However, the Grevy's zebra range has been shrinking in the north with the last stronghold being toward areas of sympatry with the plains zebra in the south (Low et al., 2008). Even if the problem of habitat loss were halted today, the low numbers of the Grevy's relative to plains zebras and the proximity of Grevy's population strongholds to areas of sympatry poses a potential risk in that it may favor the spread of hybridization. Careful monitoring will be required as management strives to solve the primary cause of declining Grevy's populations in the northern part of their range.

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