Ancient Asexuals Research Articles

Transposable elements suppress recombination in all meiotic eukaryotes, including automictic ancient asexuals: a reply to Schön and Martens

I discuss three problems with the argument that ancient asexual lineages lack meiosis and recombination resulting in a diminution of (potentially disadvantageous) transposable elements and other forms of repetitive DNA. First, all ancient asexuals are probably automictic, hence could retain meiotic recombination in the guise of gene conversion. Second, transposable elements alter recombination rates, not vice versa. Third, increasing the number of transposable elements and other forms of repetitive DNA reduces meiotic recombination rate. Ancient asexuals lack those transposable elements that are transmitted via outcrossing, and this has nothing to do with meiotic recombination.

Evidence for the evolution of multiple genomes in arbuscular mycorrhizal fungi.

Ancient asexuals directly contradict the evolutionary theories that explain why organisms should evolve a sexual life history. The mutualistic, arbuscular mycorrhizal fungi are thought to have been asexual for approximately 400 million years. In the absence of sex, highly divergent descendants of formerly allelic nucleotide sequences are thought to evolve in a genome. In mycorrhizal fungi, where individual offspring receive hundreds of nuclei from the parent, it has been hypothesized that a population of genetically different nuclei should evolve within one individual. Here we use DNA-DNA fluorescent in situ hybridization to show that genetically different nuclei co-exist in individual arbuscular mycorrhizal fungi. We also show that the population genetics techniques used in other organisms are unsuitable for detecting recombination because the assumptions and underlying processes do not fit the fungal genomic structure shown here. Instead we used a phylogenetic approach to show that the within-individual genetic variation that occurs in arbuscular mycorrhizal fungi probably evolved through accumulation of mutations in an essentially clonal genome, with some infrequent recombination events. We conclude that mycorrhizal fungi have evolved to be multi-genomic.

Rates of nucleotide substitution in sexual and anciently asexual rotifers

The class Bdelloidea of the phylum Rotifera is the largest well studied eukaryotic taxon in which males and meiosis are unknown, and the only one for which these indications of ancient asexuality are supported by cytological and molecular genetic evidence. We estimated the rates of synonymous and nonsynonymous substitutions in the hsp82 heat shock gene in bdelloids and in facultatively sexual rotifers of the class Monogononta, employing distance based and maximum likelihood methods. Relative-rate tests, using acanthocephalan rotifers as an outgroup, showed slightly higher rates of nonsynonymous substitution and slightly lower rates of synonymous substitution in bdelloids as compared with monogononts. The opposite trend, however, was seen in intraclass pairwise comparisons. If, as it seems, bdelloids have evolved asexually, an equality of bdelloid and monogonont substitution rates would suggest that the maintenance of sexual reproduction in monogononts is not attributable to an effect of sexual reproduction in limiting the load of deleterious nucleotide substitutions.

Transposable elements in sexual and ancient asexual taxa.

Sexual reproduction allows deleterious transposable elements to proliferate in populations, whereas the loss of sex, by preventing their spread, has been predicted eventually to result in a population free of such elements [Hickey, D. A. (1982) Genetics 101, 519-531]. We tested this expectation by screening representatives of a majority of animal phyla for LINE-like and gypsy-like reverse transcriptases and mariner/Tc1-like transposases. All species tested positive for reverse transcriptases except rotifers of the class Bdelloidea, the largest eukaryotic taxon in which males, hermaphrodites, and meiosis are unknown and for which ancient asexuality is supported by molecular genetic evidence. Mariner-like transposases are distributed sporadically among species and are present in bdelloid rotifers. The remarkable lack of LINE-like and gypsy-like retrotransposons in bdelloids and their ubiquitous presence in other taxa support the view that eukaryotic retrotransposons are sexually transmitted nuclear parasites and that bdelloid rotifers evolved asexually.

Evolution without sex

Genome structure confirms the chastity of some ancient asexuals.

ERRATA

EvolutionVolume 53, Issue 6 p. 2016-2016 ErrataFree Access ERRATA This article corrects the following: EVOLUTION IN A PUTATTVELY ANCIENT ASEXUAL APHID LINEAGE: RECOMBINATION AND RAPID KARYOTYPE CHANGE Benjamin B. Normark, Volume 53Issue 5Evolution pages: 1458-1469 First Published online: May 31, 2017 First published: 31 May 2017 https://doi.org/10.1111/j.1558-5646.1999.tb04586.xAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume53, Issue6December 1999Pages 2016-2016 RelatedInformation

EVOLUTION IN A PUTATTVELY ANCIENT ASEXUAL APHID LINEAGE: RECOMBINATION AND RAPID KARYOTYPE CHANGE.

Ancient asexual lineages are of great potential significance for understanding the evolutionary biology of sex, but their existence is controversial. In part, this is because claims of ancient asexuality have rested on negative evidence-a mere absence of evidence for sexuality in a taxon. M. Meselson has suggested a method, discussed by Judson and Normark (1996) and by Birky (1996), that has the potential to uncover positive evidence of ancient asexuality. Phylogenetic relationships between alleles and interallelic divergences are predicted to be very different in diploid lineages that lack recombination from those in diploid lineages that undergo recombination. I have applied Meselson's method to the putatively ancient asexual aphid tribe Tramini (Homoptera: Aphidoidea: Lachnidae), using the intron-bearing nuclear protein-coding gene elongation factor 1α (EF-1α). I found heterozygosities much lower than intraspecific divergences, indicating that some recombination has occurred, but not discriminating between recombination within an asexual lineage (automixis or mitotic recombination) and outcrossing sex. Species of Tramini (especially in the genus Trama) typically have highly structurally heterozygous karyotypes that appear to be incompatible with regular successful meiosis, and have very high levels of karyotype variability within species. I found very high levels of karyotype variability within lineages with identical EF-1α and mitochondrial (cytochrome oxidase 1 and 2) genotypes, indicating a high rate of karyotype evolution compared to the rate of nucleotide substitution.

Asexual reproduction in nonmarine ostracods

Asexual reproduction has evolved repeatedly in nonmarine ostracods and takes a variety of forms from ancient asexuals to species in which sexual and asexual lineages coexist. Clonal diversity is highly variable. There is evidence that some of this diversity is maintained by ecological differentiation. Hybridization between asexual females and males, of the same or related species, contributes to clonal diversity. Molecular data suggest that some clonal lineages are surprisingly old (more than 5 Myr). In the ancient asexual Darwinula stevensoni, from a lineage that has apparently been without sex for more than 100 Myr, a remarkable lack of sequence variation in ITS1 may be explained by occasional automixis, gene conversion or somatic recombination, or by efficient DNA repair. Overall, the ostracods provide an excellent system in which to study the evolution of reproductive modes.

DNA repair in ancient asexuals — a new solution to an old problem?

DNA repair in ancient asexuals — a new solution to an old problem?