Natural Hybrid Zones

With the lack of consensus regarding taxonomy in these studies, it may be informative to examine the dynamics of wild populations of brown lemurs at the boundaries of their parapatric distributions. Specifically, natural hybrid zones may provide insight into the history and trajectories of these neighboring but differentiated taxa. Two such brown lemur contact zones have thus far been identified: E. f. ful-vus xE. f. rufus (Lehman and Wright, 2000) and E. f. rufus xE. albocollaris (Sterling and Ramarason, 1996; Johnson and Wyner, 2000; Wyner et al., 2002).

Lehman and Wright (2000) recorded the possible presence of hybrid groups involving common brown lemur and red-fronted lemurs based on pelage characteristics during a rapid population assessment at Betsakafandrinka. Observations of E. f. fulvus and hybrids at this site are noteworthy as Betsakafandrinka lies south of the Mangoro and Oniver rivers, the putative boundary between the two subspecies (Lehman and Wright, 2000). No further investigation into this hybrid zone has been conducted. Yet, should the hybrid zone be confirmed, such intergradation between these two populations would not be surprising, considering their lack of reproductive isolation in captivity and generally close genetic affinities (Wyner et al., 1999; Pastorini et al., 2000).

The second brown lemur hybrid zone, involving red-fronted and white-collared lemurs, occurs farther south, at Andringitra National Park (Sterling and Ramarason, 1996; Wyner et al., 2002). This mountainous region is the source for several major river systems, including the Manampatrana River (noted erroneously by some to represent the E. albocollaris-E. f. rufus boundary; see Mittermeier et al., 1994; Johnson and Wyner, 2000; Irwin et al., 2005) and the Mananara River (the division between E. albocollaris and E. collaris; Tattersall, 1982; Mittermeier et al., 1994). During line-transect surveys, Sterling and Ramarason (1996) identified individuals with phenotypes of E. f. rufus primarily on the western slope, but also at higher elevations on the eastern slope. In addition, they observed some orange-bearded males (the more diagnostic sex; see below) and suspected the presence of E. collaris in this brown lemur population as well. However, the majority of males in the eastern portion of the park had the white beards typical of E. albocollaris.

These observations prompted genetic sampling of the brown lemur population on the eastern slope of Andringitra. Wyner and colleagues (Wyner, 2000; Wyner et al., 2002) analyzed sequences of mtDNA (d-loop) and nuclear markers (hemo-pexin intron 5, microsatellite 26, malic enzyme intron 8, and ceruloplasmin intron 16) to determine if E. f. rufus, E. collaris, and/or E. albocollaris diagnostic haplotypes were present in the Andringitra brown lemurs. These analyses also included a broader comparison of pure parental populations sampled elsewhere. Wyner (2000) found low mitochondrial nucleotide diversity in E. albocollaris, suggesting a relatively recent origin for this taxon as well as historically small populations—results supported by the small range (Figure 1) and presently low densities (Johnson and Overdorff, 1999; Irwin et al., 2005).

These studies also determined that the Andringitra contact zone was almost entirely composed of E. albocollaris x E. f. rufus hybrids (86% of individuals sampled) (Wyner et al., 2002). No E. collaris markers were found in the Andringitra population. Those individuals lacking hybrid haplotypes demonstrated only E. albocollaris markers, but were phenotypically indistinguishable from individuals of mixed ancestry in their area (with males, in particular, displaying a variety of pelage features, including the bushy white beards of E. albocollaris and the reddish crowns and facial stripe of E. f. rufus). Moreover, as the number of homozygotes or heterozygotes was not skewed in this population (i.e., no deviation from Hardy-Weinberg equilibrium), the hybrids likely represented multiple generations of intergradation (Wyner, 2000). Perhaps most strikingly, the hybrids of Andringitra maintained private sites (mostly in ceruloplasmin intron 16) not found in either pure populations of E. albocollaris or E. f. rufus. These results denote extensive intro-gressive hybridization in the Andringitra region (Wyner et al., 2002). Furthermore, as indicated by the apparent localized sequence evolution, the hybrid zone has likely been stable for many generations and is at least partially isolated from parental source populations (Wyner, 2000; Wyner et al., 2002).

The size of the Andringitra hybrid zone has yet to be determined. However, intermediate phenotypes have been noted as far as 32 km to the southwest (Ivohibe) from the putative center of the zone (personal observation). To the southeast, the limit may lie close to Evendra, 34 km from the center. At this site, phenotypes are not dissimilar to some of the hybrids at Andringitra (personal observation) but no hybrid haplotypes were detected in limited sampling (Wyner et al., 1999). Although the northern limit has yet to be sampled, it is likely the Andringitra hybrid zone may be at least 60-70 km in width. These dimensions are very large, particularly in relation to home ranges (a surrogate measure for dispersal) recorded for individual social groups of eastern brown lemurs (12-100 ha; Overdorff, 1991; Vasey, 1997; Johnson, 2002). More strikingly, these estimates for the hybrid zone are equivalent to more than half the length of the probable range of pure E. albocollaris (approximately 100-120 km; Figure 1).

The existence of this large hybrid zone provides some support for those who favor maintaining the taxonomic status of E. albocollaris and, by extension, E. collaris as subspecies of E. fulvus. However, the apparently old and stable contact zone is itself developing new genetic variants, evidence for the continued separation of parental populations of E. albocollaris and E. f. rufus (Wyner et al., 2002). Other differences between parental populations are also apparent, including ecology and behavior (see below). Moreover, there is no evidence for genetic exchange between E. collaris and any other brown lemur population (except for introduced E. collaris and E. f. rufus at Berenty; Jekielek, 2003). Andringitra, where no E. collaris markers were found, is the only likely site for overlap with either E. albocollaris or E. f. rufus within their original range; the Maranara River forms an effective barrier to the south and no suitable habitat remains that could facilitate E. collaris dispersal into the Andringitra headwaters region (Wyner et al., 2002). Thus, E. collaris is geographically isolated from E. albocollaris, which, in addition to the evidence for reproductive isolation from captivity, supports their elevation to full species status.

Given these complex and oft-conflicting lines of evidence, ascertaining whether white-collared and collared lemurs represent species distinct from E. fulvus is partly a semantic exercise wherein a biological continuum is arbitrarily separated into categories. Following Jolly (2001), it is perhaps more appropriate to adopt the term "allotaxa" (Grubb, 1999) for such populations that are distinct and diagnosable but may not be completely reproductively isolated. Some criteria for determining species-level designation, such as genetic distance, may not adequately reflect present intergradation or evolutionary trajectories in the brown lemur complex (see also Yoder, 2003). The biological species concept is also difficult to apply to this group, as not all populations are interfertile but reproductive isolation is independent of the degree of relatedness. In contrast, using population aggregate analysis and the phylogenetic species concept, E. albocollaris and E. collaris may be consistently diagnosed as separate species (Wyner et al., 1999). The phylogenetic species concept may also be preferred for its utility in conservation biology (Vogler and DeSalle, 1994). This method helps underscore the biological significance of critically endangered lemur taxa such as the white-collared lemur (e.g., Harcourt and Thornback, 1990; Mittermeier et al., 1992, 2005).

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