Tuesday, October 25, 2016

All-female hybrid fish species that 'uses' males for better genetics

A naturally occurring hybrid fish species, Hexagramos octogrammus/H. grampus (Hoc/Hag), composed of all-female members, is thought to have developed a unique method of evolutionary survival by switching matings between two different male species of the same genus.  The (Hoc/Hag) hybrid females are thought to have a competitive advantage because 100% of their species is capable of producing offspring, thus allowing them to quickly replicate and outnumber other species that produce both male and female offspring.  Replication without variation is an evolutionary disadvantage though, and this can be detrimental to the long term survival of the all-female species due to also developing a decreased ability to genetically adapt to environmental pressures.

Researchers from Hokkaido University in Japan have compared the genes of three species: Hexagramos octogrammus/H. agrammus (Hoc/Hag), H. octogrammus/H. otakii (Hoc/Hot), as well as their maternal pure line, H. octogrammus (Hoc).  Their study, published in Ecology and Evolution, found that the hybrid females, (Hoc/Hag), mate with their choice of two hybrid male species: (Hoc/Hot) or (Hoc/Hag).  The result of either mating always produces 100% all-female hybrid offspring in which both maternal and paternal genes influence their development.  The subsequent generation only inherits the maternal genome though, and excludes the paternal genome from gamete formation when they're able to form their own eggs.  In addition, the all female offspring can only mate with (Hoc) males whose sperm activate their eggs to start development.  The resulting (Hoc) offspring undergo normal germ cell development in which genetic recombination can occur between maternal and paternal genomes, resulting in a more diversified genome for their male/female offspring.

“When a female descendant of one of these backcrossed Hocs mates with a Hag male, a new all-female Hoc/Hag hybrid lineage arises. This could be another factor that increases the diversity of Hoc/Hac hybrids, increasing their survivability,” says the paper’s lead author, Hiroyuki Munehara.
The researchers constructed a mtDNA genealogical tree that showed that the (Hot) and (Hag) species diverged from their common ancestor around 1.5 million years ago.  Furthermore, their analysis also revealed that (Hoc/Hot) hybrids originated from hybrid (Hoc/Hag) females switching choice of host; instead of breeding with (Hag) males, they bred with larger (Hot) males that would better protect their eggs.

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