Friday, April 23, 2010

Size-advantage in sex changing fish

Did you know that many teleost fish species are sequentially hermaphroditic - starting life in one sex before switching to the other later on? When individuals start life as a male, but then become female this is called protandry; whereas females who later change to males are called protogynous.

One hypothesis for the evolution of protogyny is that in many species size provides a significant advantage to the mating success of males, but has little impact on mating outcome in females. As such, any mechanism allowing individuals to start life in female form (when they are typically small), but then "mature" into males once they have achieved large size, should be favored by natural selection. This evolutionary scenario is not nearly as implausible as it sounds because teleost fish (unlike most other vertebrates whose gonadal tissues differentiate early in development) develop their sex organs from a single, protogynous tissue type. This hypothesis for the evolution of protogyny has been dubbed the "size-advantage hypothesis."

A recent study by Erem Kazancιoǧlu and Suzanne Alonzo [2010; Evolution Accepted] uses phylogenetic comparative methods to examine the evolution of size-advantage and sequential hermaphroditism in labrid fishes: also known as the wrasses. What they find is that, indeed, the evolution of dioecy (separate sexes) from sequential hermaphroditism is relatively unlikely when the size-advantage of large males is high. However, their evidence for the evolution of protogyny from dioecious species with male size-advantage was somewhat ambiguous.

Although I enjoyed this paper quite a bit, and it seemed perfect fodder for some clever fun in photoshop (actually by E. Lu, see above), I also felt that that the study had some methodologically weak areas. For instance, the authors failed to take advantage of a new phylogenetic logistic regression procedure by Ives & Garland [2010], which seems ideally suited to their data. (In their defense, the method is brand new.) Consequently, however, the authors found themselves of the unfortunate position of using an arbitrary scoring system to estimate size-related reproductive skew: adding 1 point for the presence of "pronounced sexual dichromatism," for example, and subtracting 1 point for "alternative reproductive tactics" (which might decrease the advantage of large male size) . With a phylogenetic multivariable logistic regression, the authors could have tested for an association between the log-odds of protogyny and each of their proxies for size-based reproductive skew (which also included sexual size dimorphism, resource defense, and mate defense), while simultaneously controlling for the phylogenetic non-independence of the species in their sample.

In spite of its limitations, I found this study to be a tremendously interesting read. Due in no small part to its unusual and "sexy" subject matter, I'm sure it is destined to attract the authors considerable attention - among evolutionary biologists and lay people alike.

Wednesday, April 14, 2010

Announcement: Comparative Methods and Macroevolution In R Summer Short Course

Want to learn R? We have a short course this summer. Grad students + postdocs please apply! We have a good number of full stipends to cover all costs for the workshop in beautiful Santa Barbara, CA.

Announcement: Comparative Methods and Macroevolution In R Summer Short Course

We are pleased to announce an intensive short course on using R to perform comparative methods to be held in Santa Barbara on June 17-21. This course is funded by the National Science Foundation, and a number of stipends to cover or defray travel, room, and board are available to qualified students and post-docs. Topics covered will include an introduction to the R programming language, tree manipulation, independent contrasts and phylogenetic generalized least squares, ancestral state reconstruction, models of character evolution, diversification analyses, and community phylogenetic analysis. If you are interested please send your CV along with a short (maximum 1 page) description of your research interests, background, and reasons for taking the course. We especially encourage applications from graduate students with data sets to analyze. Please contact the co-organizers, Michael Alfaro ( and Luke Harmon ( with any questions.

Application deadline: May 15th.

Monday, April 12, 2010

Arborescence and the Rate of Evolution in Plants

A recent article by Korall et al. (2010; Evolution Online Accepted) reveals a convincing deceleration in the rate of evolution for DNA sequence associated with the origin of arborescence (tree-like growth form and life history) in ferns. This is similar to the pattern found in seed plants, where an association between the rate of sequence evolution and growth form is already known (e.g., Smith & Donoghue 2008; Soria-Hernanz et al. 2008).

In both plants and animals, simple population genetics theory predicts that for a neutrally evolving locus the rate of substitution should be equal to the per generation neutral mutation rate, μ. Since germline cells are sequestered from somatic cells in animals, and germline cells undergo a fixed number of replications that is independent of generation time, theory thus predicts that, in animals, the rate of nucleotide substitution per unit time at a neutrally evolving locus will be μ/t, for generations of length t. However, for plants the prediction is less simple. This is because in plants, germline cells are not sequestered, but are instead derived from somatic tissue. As such, germ cells in older plants should in theory have more opportunity for somatic (and thus gametic) mutation.

This means that the concomitant increase in generation time that characterizes arborescent plants is insufficient in theory to explain the decreased nucleotide substitution rates estimated empirically. The authors suggest a number of possible alternative underlying causes for this pattern. For instance, they note that both arborescent seed trees and tree ferns might share a lower rate of somatic cell replications (as suggested by Soria-Hernanz et al. 2008). This represents a fully testable hypothesis which might (in part or in whole) account for the pattern found by the authors. Alternatively, Korall et al. (2010) propose that the duration of sporophyte/gametophyte life history stages in arborescent and herbaceous plants should also be considered. This is a difficult hypothesis to test comparatively, since all arborescent species have a relatively long sporophyte phase. It might be possible to study mutation accumulation in sporophyte and gametophyte life history stages in a rapidly reproducing species under laboratory conditions.

In spite of the numerous open questions that it leaves, this article extends the relationship between arborescence and slow rates of molecular evolution beyond the seed plants, and thus into a broader group of diverse organisms. This finding will surely stimulate considerable future research.

Saturday, April 10, 2010

Goodbye...for now?

On Monday, I will begin a rotating program officer position at the National Science Foundation in the Systematic Biology and Biotic Inventories Cluster. Thus, this is going to be my last post on Dechronization until my rotation is over. It has been a really great experience being part of this blog and I've met some great people because of it. Keep up the good work Dechron'ers. I'll be reading!

P.S. The photo is of a shovel-snouted lizard (Meroles anchietae), that I got to see on a recent trip to Namibia.