New evidence for hybrid zones of forest and savanna elephants in Central and West Africa

Abstract

The African elephant consists of forest and savanna subspecies. Both subspecies are highly endangered due to severe poaching and habitat loss, and knowledge of their population structure is vital to their conservation. Previous studies have demonstrated marked genetic and morphological differences between forest and savanna elephants, and despite extensive sampling, genetic evidence of hybridization between them has been restricted largely to a few hybrids in the Garamba region of northeastern Democratic Republic of Congo (DRC). Here, we present new genetic data on hybridization from previously unsampled areas of Africa. Novel statistical methods applied to these data identify 46 hybrid samples--many more than have been previously identified--only two of which are from the Garamba region. The remaining 44 are from three other geographically distinct locations: a major hybrid zone along the border of the DRC and Uganda, a second potential hybrid zone in Central African Republic and a smaller fraction of hybrids in the Pendjari-Arli complex of West Africa. Most of the hybrids show evidence of interbreeding over more than one generation, demonstrating that hybrids are fertile. Mitochondrial and Y chromosome data demonstrate that the hybridization is bidirectional, involving males and females from both subspecies. We hypothesize that the hybrid zones may have been facilitated by poaching and habitat modification. The localized geography and rarity of hybrid zones, their possible facilitation from human pressures, and the high divergence and genetic distinctness of forest and savanna elephants throughout their ranges, are consistent with calls for separate species classification.

A comprehensive genomic history of extinct and living elephants

“…4A) and of isolation between forest and savanna elephants has implications for elephant conservation biology. While hybridization occurs between forest and savanna elephants along their current contact zone (24)(25)(26)(27), which has long hindered their recognition as distinct species (28), our genome-wide analysis shows that this process has not left detectable traces on the genomes of representative members of the two species across their range. Thus, for conservation purposes, forest elephants and savanna elephants are appropriately viewed as reproductively distinct units, meeting the definition of the Biological Species Concept (29).…”

“…Hybridization in fact still occurs locally where the two species' ranges overlap (24)(25)(26). Recent work by Mondol et al (27) shows that gene flow is bidirectional and that hybrids are fertile but appears to have not resulted in detectable introgression of nuclear alleles beyond these hybrid regions. The finding of deep population structure between the two subgroups of forest elephants (see Within-Species Analyses: Diversity, Population Size Change, and Population Substructure and Fig.…”

Habitat‐driven diversification, hybridization and cryptic diversity in the Fork‐tailed Drongo (Passeriformes: Dicruridae: Dicrurus adsimilis)

“…We suggest that this discrepancy is due to hybridization between members of D. m. atactus and the western population of D. a. divaricatus , with capture of the D. m. atactus mitochondria by D. a. divaricatus west of Lake Chad, and further that this hybridization lasted until approximately 0.5 mya (corresponding to the divergence between the two mitochondria lineages). Examples of hybridization and gene flow between taxa at the forest–savannah interface in Africa have been described for elephants (e.g., Mondol et al, ), and for birds there is also evidence for such processes in other savannah systems (Shipham, Schmidt, Joseph, & Hughes, ). It is striking that during the past 0.5 myrs the two primary D. a. divaricatu s haplotype lineages remained strongly geographically segregated and that this segregation corresponds to a previously described biogeographic barrier (Lake Chad), which suggests that the two D. a. divaricatus populations distributed on either side of the Lake Chad basin might have achieved reproductive isolation.…”

Evolutionary and demographic processes shaping geographic patterns of genetic diversity in a keystone species, the African forest elephant (Loxodonta cyclotis)

“…Hybrids between forest and savanna elephants have been documented, but only within relatively narrow transition zones between forest and savanna habitats (Comstock et al., ; Ishida et al., ; Mondol et al., ; Roca et al., ). One important point regards the number of markers needed to estimate the relative contributions of the two species to hybrid individuals.…”

“…Within the Congolian forest block, isolation by distance received strong support, as determined using both F ST (r = .86, p < .01, Mantel test) ( Figure 4b) and Rousset's distance (r = .85, p < .01, Mantel test) (Figure 4c). Hybrids between forest and savanna elephants have been documented, but only within relatively narrow transition zones between forest and savanna habitats (Comstock et al, 2002;Ishida et al, 2011;Mondol et al, 2015;Roca et al, 2001). One important point regards the number of markers needed to estimate the relative contributions of the two species to hybrid individuals.…”