Meiosis is the specialized cell division that halves cell DNA content to generate gametes, which then fused during sexual reproduction. Although meiosis is widespread, some species undergo meiosis despite lacking genes that are required for meiosis in other species. Little is known about why some species have lost meiosis genes but not meiosis. This project will investigate whether lichens, mutually beneficial coexisting fungi and algae, harbor changes in meiosis genes associated with this form of symbiosis. DNA from multiple species of fungi and algae that form lichens will be searched for gene sequences. Evolutionary analyses will compare 17 meiotic genes from multiple lichens to determine whether lichenization is associated with loss or rapid sequence change of meiosis genes.
By characterizing the effects of symbiotic coexistence upon lichenized fungal and algal reproduction, this project will provide an important comparison for other symbioses (e.g. pathogens, mutualists). This project will also further our understanding of how meiosis genes vary while the overall process remains intact. Aspects of this work will be incorporated into on-going educational activities, including high school and undergraduate student research mentoring and annual public-access seminars.
Sexual reproduction involves meiosis, the process where parent cells divide to make gametes—eggs and sperm in animals, spores in fungi—each with half the copies of each chromosome. Meiosis requires many genes and most species share a common set of meiosis genes. Meiosis and sex can be compromised if even one meiosis gene is dysfunctional. However, some species have functional meiosis despite having lost genes critical for meiosis in other species. Therefore, determining which species have lost meiosis genes without damaging meiosis is important to understanding how meiosis can sometimes vary without failing. Some fungi have the pattern described above: they undergo meiosis and sex without meiosis genes critical in other species. Which factors allow some fungi to lose meiosis genes without harmful consequences? Is it how frequently they have sexual reproduction? Is it whether they cause diseases or not? One factor could be mutualistic symbiosis, where two species coexist such that each species survives and reproduces better together. This predicts both species will evolve combinations of genes and genetic variation that work well together. If so, asexual reproduction—producing offspring identical to parents—may be best: every offspring inherits genotypes that work with genotypes of the symbiosis partner. Sex may not always be beneficial because offspring will be genetically different from parents, and their genotypes may not work well with the symbiosis partner. We investigated meiosis genes in lichens: fungi and algae in a mutualistic symbiosis. Lichens are key to understanding the evolution of sex and meiosis since: (a) both fungi and algae can potentially experience sex and meiosis; (b) conflicting pressures on sexual reproduction for species in mutualistic symbiosis could lead to changes in meiosis; (c) many fungi are known to lose meiosis genes but retain meiosis, yet few genome sequences are available to determine if lichen fungi show meiosis gene loss; and (d) no previous study investigated whether mutualistic symbiosis affects meiosis gene loss. We isolated DNA from lichens collected from the Midwest and Southeast US and used degenerate PCR to isolate and identify known meiosis genes from fungi and algae. We also surveyed publicly available genome sequences of non-lichen-forming relatives. Of multiple different species of lichenized fungi and algae found to have Rad51 (a gene coding a protein functioning in meiosis and mitosis), five had Dmc1 and/or Mnd1 (genes encoding proteins that work in a complex and function only in meiosis). Some of these species are not known sexuals, so our findings predict that meiosis happens under unknown conditions. Another five species did not have detected Dmc1 or Mnd1. To efficiently study more meiosis genes within a single lichen, "NextGen" genome sequencing was performed on a lichen containing the fungus Physcia and algae Trebouxia. Initial data assemblies were prepared and the best version of the Physcia/Trebouxia genomes will be made publicly accessible. Trebouxia was a "typical" green alga: it had the same core set of meiosis genes in non-lichenized green algae. Physcia had most of the meiosis genes as non-lichenized fungi, but did not have Pch2 or Hop1. These genes have been independently lost together in several non-lichenized fungi; future work can explore what properties Physcia has in common with species that lost these genes and whether their meiosis changed. We started work applying the same "NextGen" sequencing to several lichenized Xanthomendoza fungi, but on-going troubleshooting will delay release of the genome sequences to the public. Many lichenized fungi and algae seem to have the same set of meiosis genes as non-lichenized relatives or, forPhyscia, have independently lost meiosis genes following the same patterns as in non-lichenized relatives (i.e., Pch2 and Hop1 lost together). However, having the same genes could still mean that these genes change at a faster or slower rate in lichen species. We compared evolutionary rates in Rad51, Dmc1, and Mnd1 in lichenizedvs. non-lichenized fungi; different classes of fungi showed different rates, but lichenized fungus genes were not consistently faster or slower evolving than other fungi. Thus, mutualistic symbiosis in lichens may not explain why some fungi tolerate meiosis gene loss. Nevertheless, our sequences can be resources for future investigations into other factors that influence meiosis gene variation in fungi (frequency of sex, ability to self-fertilize, etc.). Continuing to study natural variation in meiosis will contribute an understanding of how individual meiosis genes can lose function and yet meiosis and sexual reproduction can persist. The general appeal and relevance of sex in biology makes this work interesting to a broad audience; thus, this work has been featured in multiple research seminars (internal and external) by the PI and co-PI. Lichens and meiosis have also been the inspiration for a series of original musical pieces written and performed by the co-PI as part of the Iowa City Darwin Day events in 2013 and 2014.
