gcbias

Yaniv and I’s paper on meiotic drive and the evolution of female recombination rates has just appeared in the latest Genetics. You can read more about the paper in our blog post here.

Carl, Peter, and I’s paper on the power of comparative methods just appeared in Evolution. The paper is ArXived here with code here, Carl has slides and a blog post (here and here). As you can see from the number of links Carl is a big proponent of open science, in fact he kept an open lab notes throughout this work.

Comparative methods remain one of the most useful ways for us to understand the adaptive and ecological significance of particular phenotypes. Applications of this methods rely on phylogenies in order to distinguish between convergence (giving us natural experimental replication) and common descent. These methods also given us many useful insights into the tempo and mode of phenotypic change.

To understand rates of phenotypic evolution on phylogenies and provide the tools for the comparative method a range of increasing complicated models of trait evolution have been developed incorporating varying rates of evolution etc. However, the amount of information present in these data is limited as the traits at the tips of a tree are potentially highly correlated due to the underlying phylogeny. This means that we should be careful to understand the limitations of these data to distinguish between complex models, and also acknowledge that many of the assumptions that we often use in model fitting and hypothesis choice are not valid (e.g. our asymptotic assumptions). In this paper we produce a general framework for exploring these issues through simulations. A lot of this work was driven forward by Carl’s desire to really understand the validity of the assumptions underlying the application of these methods, so we hope that this will get a lot of use in the future in guiding the design and analysis of these types of data.

Yaniv and I’s paper: “Scrambling eggs: Meiotic Drive and the Evolution of Female Recombination Rates” is out on the Genetics preprint server (here) and the arXived versions of our paper are available here.

Conflict is everywhere during sexual reproduction. Even meiosis is a battlefield (Pat Benatar had that right), as during meiosis and gametogenesis alleles can complete with each other within an individual to ensure their transmission to the next generation. No where is this more apparent than during female meiosis, where out of the 4 products of meiosis only one will go on to form the egg. This allows an opportunity for (true) meiotic drive, as an allele that distort meiosis in its favor when heterozygous can spread through the population. One great example of female meiotic drive is the chromosome knob system in maize (see here and references within), another putative example is provided by the great work of Fishman and Willis in Mimulus (here and here). There’s even female meiotic drive alleles humans, by chromosomal fusions, but the deleterious consequences in males mean that they only persist for short time periods.

Such selfish drive systems set up the arena for further conflict, especially if they have deleterious consequences. Alleles that facilitate this drive can also spread if they team up with the meiotic drive alleles. While alleles that suppress the drive can spread as they benefit from the reduced transmission of the deleterious driver to their descendents.

In our latest paper, Yaniv and I explore how models of meiotic drive may have shaped sex differences in recombination rates. In many systems recombination is an essential part of meiosis, with at least one crossover required per chromosome to ensure correct segregation. Perhaps surprisingly then recombination rates evolve very rapidly and the sexes often differ dramatically in their number of recombination events and their positions. For example in humans ~26 events are placed down during male meiosis but around 40 are placed down in female meiosis. This pattern of sex differences in recombination rates is wide-spread, with female recombination rates often -but not always- being higher than male rates. Yaniv and I suggest that this could be due to the role of female recombination in facilitating or ameliorating female meiotic drive. The details are a bit involved, as the predictions vary depending on which stage of meiosis drive alleles take advantage of. But the basic idea is that because the centromere plays such a key role in meiosis and recombination determines whether alleles segregate with their, or their non-sister centromere, changes to the female recombination can have powerful effects on meiotic drive and be selected on as a consequence.

We see this work as part of the emerging picture that conflict may be common in meiosis and that various features of meiosis, from the complexity of centromeres, to the stages of meiosis, to sex itself may have been profoundly shaped by this struggle (Burt and Trivers’s book is a great place to read about this). These ideas has been around for a long time and have a lot to recommend them, indeed the idea that drive may affect recombination rates has been around for a while (see Thomson and Feldman, Haig and Grafen), however, the sex-specific context had received little attention (but see Haig 2010, and Lenormand 2003). Strangely these conflict ideas are often ignored by researchers interested in recombination and its evolution, who seem to prefer to focus on more traditional explanations for sex and recombination due to recombination role in facilitating or hindering selection.

The rapid evolution of many components of meiosis is certainly consistent with these types of conflict hypotheses, and our own work on PRDM9 suggests that even the fine-scale features of recombination may be involved in another facet of these types of conflict. We have a long way to go in understanding the causes and consequences of these conflicts, but hopefully advances in sequencing technology will allow us to investigate distortion and recombination in meiosis in fuller detail.

[UPDATED: Thanks to Jeff Ross-Ibarra for edits]