Alexander Brandt

Welcome to my mollilab profile! I am Alex, third year PhD student and interested in unraveling effects of ecology and live history traits (especially the absence of sex) on genome evolution using high troughput sequencing techniques and Linux command line fiddling. If you are interested in a short outline about what I did during my PhD so far and what I am still doing in order to, one day, kiss the Gaenseliesel on her cheek, you are dead right here ?

I) Deleterious mutation accumulation in asexual oribatid mites?

It is established consensus among evolutionary biologists (based on theory and several lines of empirical evidence) that obligately asexual lineages accumulate slightly deleterious mutations and, hence, go extinct in the long-term. We wanted to know if asexual oribatid mites, which lost sex tens of millions of years ago, show signatures of deleterious mutation accumulation. Surprisingly, we found less accumulation of slightly deleterious nonsynonymus and synonymous mutations in asexual as compared to sexual oribatid mites. Further, nonsynonymous mutations in asexual mites were less strongly deleterious. One possible explanation for this could be that asexual oribatid mites have larger populations as compared to their sexual relatives, a pattern that is indeed observed in natural populations. For more information, check this publication or the Göttingen and Lausanne press-releases.

II) Genomic consequences of asexuality in Timema stick insects

Contrasting our findings in asexual oribatid mites, asexual Timema stick insects display a variety of predicted consequences for genome evolution under asexuality. Timema stick insects are an outstandingly well suited system for analysing various aspects of parthenogenesis because the genus comprises several independent transitions to asexuality, leaving five pairs of direct sex-asex sister lineages. In asexual Timema we found higher rates of GC-biased gene conversion, lower levels of neutral polymorphism and less effective purifying selection as inferred by nonsynonymous vs. synonymous divergence within and between species. Interestingly, asexual Timema lineages display higher rates of synonymous divergence as compared to their sexual sisters (likely not due to less effective selection on codon usage bias) and a higher number of co-orthologous loci. For more information, see the resulting publication (first author: Jens Bast):

III) Again deleterious mutations: What about other asexual insects?

Consequences of asexuality on genome evolution in oribatid mites and Timema stick insects obviously do not converge and studies done in a number of other organisms are not conclusive, either. Hence, we further analysed the effectiveness of purifying selection in a variety of asexual and sexual hexapod species, covering eight major hexapod groups using transcriptome data of the 1K Insect Transcriptome Evolution (1kite) project. Our results, here, have to be interpreted with care because the loss of sex does not necessarily converge with the split of sexual and asexual lineages in the underlying phylogeny used for analyses of nonsynonymous vs. synonymous divergence. However, we found no evidence for less effective purifying selection acting at nonsynonymous and synonymous sites and no evidence for increased deleteriousness of nonsynonymous mutations in asexual insects. The manuscript will be uploaded soon to bioRxiv!

IV) Evidence for ancient asexuality in oribatid mites: revival of the Meselson effect?

This one will form the last project within the scope of my PhD. According to several independent lines of evidence, many oribatid mites are 1) obligately asexual and 2) lost sex millions of years, ago. However, as most populations of asexual oribatid mites also comprise very low numbers of males, their status as „ancient asexual scandals“ is, still, questioned. Here, we aim at answering the question of rare sex in oribatid mites and, if possible, dating their loss of sex by analysing allelic sequence divergence (the so called Meselson effect). The Meselson effect predicts very high allelic sequence divergence within individuals (higher than between individuals and populations) in ancient asexual lineages because alleles are supposed to accumulate synonymous substitutions independently and diverge more and more after sex is lost. This also leaves a very specific topological signature in the respective haplotype phylogeny. We will search for these topological signatures in different asexual oribatid mite lineages and, if present, date the initial split of the haplotypes using molecular clocks.