Differences in homologous recombination and maintenance of heteropolyploidy between Haloferax volcanii and Haloferax mediterranei

Abstract

Polyploidy, the phenomenon of having more than one copy of the genome in an organism, is common among haloarchaea. While providing short-term benefits for DNA repair, polyploidy is generally regarded as an evolutionary trap that by the notion of the Mullers ratchet will inevitably conclude in the species decline or even extinction due to a gradual reduction in fitness. In most reported cases of polyploidy in archaea, the genetic state of the organism is considered as homoploidy i.e. all copies of the genome are identical. Here we demonstrate that while this is indeed the prevalent genetic status in the halophilic archaeon H. volcanii, its close relative H. mediterranei maintains a prolonged heteroploidy state in a non-selective environment once a second allele is introduced. Moreover, a strong genetic linkage was observed between two distant loci in H. mediterranei indicating a low rate of homologous recombination while almost no such linkage was shown in H. volcanii indicating a high rate of recombination in the latter species. We suggest that H. volcanii escapes Mullers ratchet by means of an effective chromosome- equalizing gene-conversion mechanism facilitated by highly active homologous recombination, whereas H. mediterranei must elude the ratchet via a different, yet to be elucidated mechanism.

An archaeal Cas3 protein facilitates rapid recovery from DNA damage

“…To obtain more direct results about the role of cas3 in HR pathway, we decided to execute a “pop-in” recombination assay (Dattani et al 2022 ), which enabled us to measure the integration efficiency of a “suicide” plasmid (that does not contain a H. volcanii origin of replication), into the Δcas3 mutant genome, which could be achieved only via HR of the plasmid with the chromosome.…”