In the absence of reproductive isolation – Extensive gene flow after speciation

Abstract

In the biological species concept, much of the genomes cannot be exchanged between species 1,2 . In the modern genic view, species are distinct as long as genes that delineate the morphological, ecological and reproductive differences remain distinct 2 . The rest (or the bulk) of the genomes should be freely interchangeable. The core of the species concept therefore demands finding out the full potential of introgressions between species. In a survey of two closely related mangrove species (Rhizophora mucronata and R. stylosa) on the coasts of the western Pacific and Indian oceans, we found that the genomes are well delineated in allopatry, echoing their morphological and ecological divergence. The two species are sympatric/parapatric in the Daintree River area of northeastern Australia. In sympatry, their genomes harbor 7,700 and 3,100 introgression blocks, respectively, with each block averaging about 3-4 Kb. These fine-grained and strongly-penetrant introgressions suggest that each species must have evolved many differentially-adaptive (and, hence, non-introgressable) genes that contribute to speciation. We identify 30 such genes, seven of which are about flower development, within small genomic islets with a mean size of 1.4 Kb. In sympatry, the species-specific genomic islets account for only a small fraction (< 15%) of the genomes while the rest appears interchangeable.

Sympatric or micro-allopatric speciation in a glacial lake? Genomic islands support neither

“…3F ) for GS and GE. Moreover, recent strong empirical evidence on sympatric mangroves revealed that significant post-speciation gene flow resulted in a large number of introgression blocks averaging only about 3–4 kb in size and non-introgressable genomic islands averaging 1.4 kb in size [ 69 ]. Both theoretical and empirical results indicated that large genomic islands are less likely to form in sympatric species (Fig.…”