Gene flow between vicariant tree species: insights into savanna-forest evolutionary relationships

Abstract

Studying the genetic structure of vicariant species (i.e., closely related species that occupy ecologically distinct yet adjacent habitats) can shed light on the evolution and divergence of species with different ecological requirements. A previous phylogeographic study identified chloroplast DNA haplotype sharing between two vicariant tree species, one from forest (Hymenaea courbaril) and one from savanna (H. stigonocarpa) habitats. These species co-occur in the Brazilian Cerrado, a biome that encompasses forest patches and riverine forests within a savanna matrix. In order to investigate the evolutionary processes involved in the genetic divergence of these trees, we used nuclear microsatellite markers, statistical methods including approximate Bayesian computation (ABC), and leaf morphology to analyze neighboring and distant populations. Bayesian analysis revealed admixture between the species. ABC analysis supported the scenarios with the occurrence of gene flow between species during the Last Glacial Maximum or from the Holocene to the present, when compared to alternative scenarios of no gene flow or constant gene flow since divergence. However, putative hybrids did not exhibit intermediate leaflet morphology, which could be related to distinct selective pressures maintaining species integrity even in the face of gene flow. Our results suggest that despite morphological differences between savanna and forest species, interspecific barriers to gene flow might not be fully developed between vicariant tree species and that interspecific hybridization in trees from Cerrado biome may be an underdiagnosed process.

GENETIC DIVERSITY OF Hymenaea courbaril L. IN THE MATO GROSSO AMAZON: IMPLICATIONS FOR CONSERVATION

“…The reproductive system of a species influences the distribution of genetic variation among and within populations. In H. courbaril, where gene are transmitted from one generation to another through cross-pollination, Resende-Moreira et al (2017) observed high intrapopulation genetic diversity (>60%).…”

Hybrid zone of a tree in a Cerrado/Atlantic Forest ecotone as a hotspot of genetic diversity and conservation

“…Gene flow in vicariant species such as P . reticulata with subspecific taxa typical from forests and savannas was also identified in a few other tree species in savanna–forest boundaries including ecotonal and riparian forests in the Cerrado biome (Cavallari et al, 2010; Muniz et al, 2020; Resende‐Moreira et al, 2017). This can suggest that hybridization in the ecotonal region between Atlantic Forest and Cerrado can be a relevant evolutionary process increasing the genetic diversity of the species of these biomes.…”

“…The high genetic divergence between ecotypes accounting for 22.2–33.7% of the total genetic variance (Table ) raises questions about the taxonomy of the genus. In comparison, divergence between two Hymenaea (Fabaceae) tree species from the Atlantic Forest and Cerrado accounted for 26% of the total genetic variance based on seven nuclear microsatellites (mean H E = 0.600, Resende‐Moreira et al, 2017). In Eucalyptus globulus , conversely, differentiation between ecotypes was lower, with pairwise F ST values ranging from 0.03 to 0.21 based on using 12 nuclear microsatellites (mean H E = 0.84, Foster et al, 2007).…”

“…This spatial proximity can provide opportunities for gene flow between closely related species if reproductive isolation between these taxa is not completed. In fact, a few studies using nuclear markers with sampling in contact zones between the Atlantic Forest and the Cerrado have found gene flow between closely related tree taxa (Cavallari et al, 2010;Lacerda et al, 2002;Muniz et al, 2020;Resende-Moreira et al, 2017).…”