Species delimitation in the African tree genus Lophira (Ochnaceae) reveals cryptic genetic variation

Abstract

Species delimitation remains a crucial issue for widespread plants occurring across forest-savanna ecotone such as Lophira (Ochnaceae). Most taxonomists recognize two parapatric African tree species, widely distributed and morphologically similar but occurring in contrasted habitats: L. lanceolata in the Sudanian dry forests and savannahs and L. alata in the dense Guineo-Congolian forests. Both species co-occur along a ca. 3000 km long forest-savanna mosaic belt, constituting ideal models for investigating hybridization patterns and the impact of past glacial periods on the genetic structures in two types of ecosystems. We genotyped 10 nuclear microsatellites for 803 individuals sampled across the distribution range of Lophira. Both species exhibit similar levels of genetic diversity [He = 0.52 (L. alata); 0.44 (L. lanceolata)] and are well differentiated, consistent with taxonomic delimitation (FST = 0.36; RST = 0.49), refuting the hypothesis that they might constitute ecotypes rather than distinct species. Furthermore, L. alata displayed two deeply differentiated clusters (FST = 0.37; RST = 0.53) distributed in parapatry, one endemic to Western Gabon while another cluster extended over the remaining species range, suggests that L. alata is made of two cryptic species. We showed that rare hybrids occur in some contact zones between these three species, leaving a weak signal of introgression between L. lanceolata and the northern cluster of L. alata. At the intra-specific level, the latter species also show weak genetic structuring between Upper and Lower Guinea and the intensity did not differ strikingly between rainforest and savanna ecosystems. The discovery of a new species of Lophira with a narrow distribution in West Gabon where it is intensively exploited for its timber requires to evaluate its conservation status.