The brown lemur group varies in karyotypes and chromosome morphology, and these characters may be used to distinguish the three species. All E. fulvus subspecies share a common diploid number (2N = 60) and have similar morphology in G-, Q-, and R-banding patterns (Rumpler, 1975; Hamilton and Buettner-Janusch, 1977; Hamilton et al., 1980). In contrast, E. collaris is polymorphic in karyotype (2N = 50, 51, 52), with 2N = 51 individuals (with a heteromorphic metacentric/acrocentric pair) likely resulting from hybridization between individuals with the other karyomorphs (Buettner-Janusch and Hamilton, 1979). E. albocollaris maintains a diploid number of 48; this distinction was the basis for the separation of this taxon from the closely related E. collaris (Rumpler, 1975). It has been proposed that E. collaris and E. albocollaris diverged from an ancestral E. fulvus karyotype via the addition of four and six pairs, respectively, of metacentrics (through Robertsonian fusion of acrocentrics), two of which are shared between the two species (Hamilton et al., 1980; Rumpler, 1989).
Similarity in chromosome complements in these groups does not necessarily correspond with reproductive compatibility. As expected, the E. fulvus subspecies are fully interfertile in captivity (Rumpler, 1989; Tattersall, 1993). Moreover, both E. collaris and E. albocollaris produce fertile hybrids when each is crossed with E. fulvus subspecies (2N = 60) (Rumpler, 1975; Buettner-Janusch and Hamilton, 1979; Hamilton et al., 1980). However, captive E. albocollaris x E. collaris hybrids are sterile, with meiotic chromosomes arranged in long chains of six (Rumpler, 1990, in Tattersall, 1993). Djlelati et al. (1997) noted severe reproductive breakdown in E. albocollaris x E. collaris hybrids, citing autosomal-sex chromosomal associations as a cause for impairment of spermatogenesis. Results from this study also support the more recent common ancestry of these two taxa, their karyotypes having developed through different rearrangements of the chromosomes found in an intermediary ancestor (Djlelati et al., 1997). With reproductive isolation apparent in these groups, Djlelati et al. (1997) proposed promoting these taxa to separate species status (E. albocollaris and E. collaris).
Recent studies in molecular genetics have also examined the brown lemur spe-ciation problem (Wyner et al., 1999; Wyner, 2000; Pastorini et al., 2000). Wyner et al. (1999) investigated evolutionary relationships among the six brown lemur taxa by sequencing mitochondrial DNA (including d-loop, 12S rRNA, and cytochrome b) and nuclear DNA (casein kinase II) regions. These authors used population aggregate analysis to segregate the brown lemurs into three distinct evolutionary units: a collared lemur unit (supported by three diagnostic sites), a white-collared lemur unit (with six diagnostic sites), and a group comprised of the four E. fulvus subspecies (supported by two diagnostic sites, but with no markers for distinguishing individual subspecies). Based mainly on phylogenetic species concept principles, the findings of Wyner et al. (1999) support the proposal by Djlelati et al. (1997) to elevate the two southeasternmost brown lemur populations to full species status.
Pastorini et al. (2000), however, arrived at opposite conclusions in their study of brown lemur phylogeny. These authors sequenced part of the COIII gene, all of ND3, ND4L, and ND4 genes, and five tRNA genes. White-collared and collared lemurs did indeed form a clade: pairwise distances for these two taxa were among the lowest of subspecies comparisons, and distances were relatively large between this lineage and any other distinguished E. fulvus population. Yet because pairwise distances were much greater between E. fulvus and the outgroup E. macaco, combined with the evidence for some continued interfertility (see above), Pastorini et al. (2000) opted to retain collared and white-collared lemurs as E. fulvus subspecies. Four additional brown lemur clades were also identified, but they did not segregate the traditionally recognized subspecies. E. f. rufus was sorted into two separate clades (not based on the recent east-west split, but instead on a north-south split in western populations) while E. f. fulvus also appeared in two groups (one of which also included E. f. sanfordi and E. f. albifrons). Interestingly, Wyner et al. (1999) were also unable to identify markers to distinguish the traditionally recognized four northern E. fulvus subspecies. Despite this and the potentially polyphyletic nature of the E. fulvus clades delineated by Pastorini et al. (2000), collared and white-collared lemurs consistently sorted together in both studies, to the exclusion of other brown lemurs, lending support to their taxonomic separation from E. fulvus (see also Yoder and Irwin, 1999, and Wyner et al., 2000, for similar results with reduced sampling).
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