OB  

EVOLUTIONARY GENETICS

OF CICHLID FISHES

 
  
 
 
 
 

 
 

Reade B. Roberts

Postdoctoral Fellow, Department of Biology

BS Biochemistry, Susquehanna University, 1997

PhD Cell & Developmental Biology, Vanderbilt University, 2003

Full CV here

 

 

Research

My ultimate research interest is to understand genetic complexity and how it determines phenotype, in a natural, evolutionary context. The hundreds of cichlid species of Lake Malawi provide an excellent model system to explore this interest. In addition to the startling wealth of phenotypic diversity available for study, the genetic structure of the cichlid flock lends itself to a number of complementary strategies for genetic analysis. The cichlid species are closely related and capable of hybridization, and it appears that many phenotypes and their underlying genotypes are shared in various combinations across the flock. Lab-reared family studies can be combined with association mapping within and across natural populations to pinpoint the genetic basis of adaptive traits, while providing important evolutionary context.

Pigmentation

Pigmentation has classically been a starting point for developing genetic resources in model organisms, and to study the details of neural crest development, patterning, and regeneration. The Malawi cichlid flock is an untapped “natural mutagenesis screen” for pigmentation genes, with an astounding diversity of color pattern. Much of this diversity is associated with ornate male nuptial color, which often delineates species boundaries. That said, within species there can be significant diversity in pigmentation across geographically isolated populations. Also, most species show strong sexual dimorphism in pigment phenotypes, with females usually expressing cryptic coloration. Beyond providing numerous easily identifiable traits for study, each level of phenotypic diversity provides additional avenues to study how genetic complexity evolves to control phenotype.

Recently, I exploited the genetic structure of the flock to map Pax7 as the gene underlying the "orange blotch" phenotype, a color morph found in multiple genera across the lake. The orange blotch morph is cryptic, found primarily in females, and disrupts male nuptial coloration. Combined with our work exploring the genetic basis of sex determination, the study of orange blotch provided important empirical evidence for models of the evolution of sex determination via sexual conflicts.

I also determined genetically that the orange blotch morph evolved once within the flock, yet the orange blotch allele of Pax7 produces a range of color morphs across and within populations. I am currently interested in the genetic control of this variation, and am examining allelic versus genomic background effects on phenotype. Since Pax7 is an important developmental gene separate of its role in pigmentation, it will be interesting to understand how the orange blotch allele evolved without producing negative pleitropic consequences in other tissues. Beyond orange blotch, we are combining interspecific crosses and association studies across natural populations to identify genes controlling a number of male nuptial color traits.

Sex determination

In contrast to essentially uniform and evolutionarily stable genetic sex determination systems found in mammals and birds, fishes have a wide diversity of genetic sex determination systems. The cichlids appear to have particularly rapid evolution of sex determination. In addition to the several known sex determination loci in tilapiine cichlids, we recently identified two additional loci that determine sex in the Malawi cichlids, one tightly linked to the orange blotch allele of Pax7. Interestingly, these two sex determiners are found segregating within populations and families, producing sex ratios and epistatic interactions considered unlikely by classical models of sex determination evolution. From extensive family studies we also infer the existence of still additional genetic sex determiners in the Malawi cichlids. We are currently attempting to pinpoint the causative polymorphisms underlying the known sex determination loci, as well as map the additional inferred loci.


jaws

Morphological evolution

Most recently I identified alleles of the Ptch1 gene linked to subtle differences in lower jaw morphology associated with divergent feeding strategies. While these alleles are fixed in some species and contribute to genus-specific morphology, in some single-species populations they continue to segregate. Thus, the same alleles that help to define macroevolutionary divergence among genera also contribute to continuing microevolutionary fine-tuning of adaptive traits within some species. Studying loci of relatively minor effect introduces new challenges relative to the binary traits that have been mapped in cichlids and other natural evo-devo model systems; however, it seems likely that such minor changes have been responsible for the bulk of evolutionary adaptation. Ultimately, I believe the cichlid system provides the right mix of natural population structure and tractability in the lab to allow us to identify and understand these changes. With several cichlid fish genomes just recently released, the system will be only more powerful to examine the genomics of adaptation and speciation.

Recent publications

Roberts RB, Hu Y, Albertson RC, Kocher TD. 2011 Craniofacial divergence and ongoing adaptation via the hedgehog pathway. Proceedings of the National Academy of Sciences 108: 13194-13199. (link)

Ser J, Roberts RB, Kocher TD. 2010 Multiple interacting loci control sex determination in Lake Malawi cichlid fishes. Evolution 64: 486-501. (PDF)

Roberts RB, Ser J, Kocher TD. 2009 Sexual conflict resolved by invasion of a novel sex determiner in Lake Malawi cichlids. Science 326: 998-1001. Access article free!

In the news:

The pattern of a female sex chromosome. Madge B. 2009. Cell 139: 445. (PDF)

Boys against girls. 2009. Nature 641: 700. (PDF)

East African cichlid fish offer new understanding of genetic basis of sex determination. 2009. National Science Foundation News.

Genetic conflict in fish led to evolution of new sex chromosomes. Blake KE. 2009. UMD College of Chemical and Life Sciences.

Personal interests

In my time away from the lab I am involved in racing greyhound rescue through the group Greyhound Welfare. In addition to my own two adopted ex-racers, I usually have a foster grey at any given time, and spend many weekends at open houses to promote adoption and awareness of this great breed. I am also an avid collector of vinyl records across genres, and am always looking for new discoveries.


Etta
Etta the greyhound relaxing in front my record collection.

 
 
 
 

Reade

Various morphs of Pax7-OBOB morphs

A small sample of pigmentation in Malawi cichlidsPigment patterns
Above images by Reade Roberts

OB morphs
Evolution cover image by Ad Konings