High selfing rate and inbreeding depression correlate with past range expansion in an endangered African timber species Pericopsis elata

Self-fertilization and inbreeding depression (δ) are expected to co-evolve, particularly during range expansion, yet few studies have examined this dynamic in tropical trees, which could be relevant for their conservation. We investigated the mating system of Pericopsis elata (Fabaceae), an endangered timber species with a fragmented distribution in Central Africa comprising two distinct genetic clusters: the Sangha River Interval (SRI) and the eastern cluster (Congolia). The SRI has recently expanded its range. We hypothesized that this expansion influenced both selfing rates and δ. Using microsatellite markers, we genotyped 54 mother trees and their offspring across five populations. Results revealed exceptionally high selfing rates in the SRI (mean = 0.88), significantly exceeding those in the eastern cluster (0.54). Within the SRI, selfing rates were consistent (0.84-0.92), while δ remained high (0.82-0.97), except at the expansion front (0.37). These findings suggest that past range contraction and fragmentation may have favored selfing for reproductive assurance, albeit without further increasing selfing rates during the recent expansion. However, the persistence of strong δ despite high selfing rates challenges classical expectations of purging deleterious alleles. This study underscores the need to consider local variation in mating systems when developing conservation strategies for P. elata. More broadly, it provides rare empirical evidence of intraspecific mating system variation in tropical trees with high selfing rates without necessarily reducing δ. From an applied perspective, plantations of P. elata would benefit from outcrossed seeds, though seed number and position within pods were not correlated with outcrossing rate.