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Hybrid Genomes in Eukaryotes

By Bbenzon @bbenzon
#PLOSGenetics: Eukaryote hybrid genomes — Bill Benzon (@bbenzon) December 5, 2019
Runemark A, Vallejo-Marin M, Meier JI (2019) Eukaryote hybrid genomes. PLoS Genet 15(11): e1008404.
Interspecific hybridization is the process where closely related species mate and produce offspring with admixed genomes. The genomic revolution has shown that hybridization is common, and that it may represent an important source of novel variation. Although most interspecific hybrids are sterile or less fit than their parents, some may survive and reproduce, enabling the transfer of adaptive variants across the species boundary, and even result in the formation of novel evolutionary lineages. There are two main variants of hybrid species genomes: allopolyploid, which have one full chromosome set from each parent species, and homoploid, which are a mosaic of the parent species genomes with no increase in chromosome number. The establishment of hybrid species requires the development of reproductive isolation against parental species. Allopolyploid species often have strong intrinsic reproductive barriers due to differences in chromosome number, and homoploid hybrids can become reproductively isolated from the parent species through assortment of genetic incompatibilities. However, both types of hybrids can become further reproductively isolated, gaining extrinsic isolation barriers, by exploiting novel ecological niches, relative to their parents. Hybrids represent the merging of divergent genomes and thus face problems arising from incompatible combinations of genes. Thus hybrid genomes are highly dynamic and undergo rapid evolutionary change, including genome stabilization in which selection against incompatible combinations results in fixation of compatible ancestry block combinations within the hybrid species. The potential for rapid adaptation or speciation makes hybrid genomes a particularly exciting subject of in evolutionary biology. Here we summarize how introgressed alleles or hybrid species can establish and how the resulting hybrid genomes evolve.
Genetic exchange between species can impede the evolution of biodiversity because gene flow between diverging species counteracts their differentiation and hybridization between recently diverged species can lead to loss of genetic adaptations or species fusion[1]. Traditionally, zoologists have viewed interspecific hybridization as maladaptive behaviour[2] which can result in breaking up co-adapted gene complexes[3]. In contrast, plant biologists recognized early on that hybridization can sometimes be an important evolutionary force, contributing to increasing biodiversity[4][5]. Recently, evidence has been accumulating showing that hybridization is also an important evolutionary process in animals[1][6][7]. Interspecific hybridization can enrich the genetic diversity of introgressed taxa, lead to introgression of beneficial genetic variation or even generate new hybrid species[1]. Hybridization is now also known to contribute to the evolutionary potential in several textbook examples of adaptive radiation, including the Geospiza Galapagos finches[8], African cichlid fishes[9], Heliconius butterflies[10][11][12] and Hawaiian Madiinae tarweeds and silverswords[13]. Here we review the evolutionary outcomes of interspecific hybridization and the properties of genomes of hybrid genomes. Many of the discussed topics also apply to hybridization between different subspecies or populations of the same species, but here we focus on interspecific hybridization (referred to as hybridization in this review).

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