Interspecific hybridization and island colonization history, not rarity, most strongly affect the genetic diversity in a clade of Mascarene-endemic trees

Echoes of ancient introgression punctuate stable genomic lineages in the evolution of figs

Studies investigating the evolution of flowering plants have long focused on isolating mechanisms such as pollinator specificity. Some recent studies have proposed a role for introgressive hybridization between species, recognizing that isolating processes such as pollinator specialization may not be complete barriers to hybridization. Occasional hybridization may therefore lead to distinct yet reproductively connected lineages. We investigate the balance between introgression and reproductive isolation in a diverse clade using a densely sampled phylogenomic study of fig trees (Ficus, Moraceae). Codiversification with specialized pollinating wasps (Agaonidae) is recognized as a major engine of fig diversity, leading to about 850 species. Nevertheless, some studies have focused on the importance of hybridization in Ficus, highlighting the consequences of pollinator sharing. Here, we employ dense taxon sampling (520 species) throughout Moraceae and 1,751 loci to investigate phylogenetic relationships and the prevalence of introgression among species throughout the history of Ficus. We present a well-resolved phylogenomic backbone for Ficus, providing a solid foundation for an updated classification. Our results paint a picture of phylogenetically stable evolution within lineages punctuated by occasional local introgression events likely mediated by local pollinator sharing, illustrated by clear cases of cytoplasmic introgression that have been nearly drowned out of the nuclear genome through subsequent lineage fidelity. The phylogenetic history of figs thus highlights that while hybridization is an important process in plant evolution, the mere ability of species to hybridize locally does not necessarily translate into ongoing introgression between distant lineages, particularly in the presence of obligate plant-pollinator relationships.

Multiple colonizations and hybridization of a freshwater fish group on a satellite island of Sulawesi

Repeated colonizations and resultant hybridization may increase lineage diversity on an island if introgression occurs only in a portion of the indigenous island lineage. Therefore, to precisely understand how island biodiversity was shaped, it is essential to reconstruct the history of secondary colonization and resultant hybridization both in time and space. In this study, we reconstructed the history of multiple colonizations of the Oryzias woworae species group, a freshwater fish group of the family Adrianichthyidae, from Sulawesi Island to its southeast satellite island, Muna Island. Phylogenetic and species tree analyses using genome-wide single-nucleotide polymorphisms revealed that all local populations on Muna Island were monophyletic, but that there were several genetically distinct lineages within the island. Population structure and phylogenetic network analyses demonstrated that colonization of this island occurred more than once, and that secondary colonization and resultant introgressive hybridization occurred only in one local population on the island. The spatially heterogeneous introgression induced by the multiple colonizations were also supported by differential admixture analyses. In addition, the differential admixture analyses detected reverse colonization from Muna Island to the Sulawesi mainland. Coalescence-based demographic inference estimated that these mutual colonizations occurred during the middle to late Quaternary period, during which sea level repeatedly declined; this indicates that the colonizations occurred via land bridges. We conclude that these mutual colonizations between Muna Island and the Sulawesi mainland, and the resultant spatially heterogeneous introgression shaped the current biodiversity of this species group in this area.