The Cave Biology Research Group (CBRG) at New York University
contains evolutionists, developmental biologists, conservationists and educators. Current projects include comparative genomic mapping to find the genes responsible for eye regression and depigmentation in different populations of cave fishes, phylogenetic studies, and educational projects. For information about the CBRG, e-mail to rb4@scires.nyu.edu
Details of the research rationale for the laboratory of Richard Borowsky (Department of Biology, NYC, NY 10003) are as follows:
Research Interests and Rationale:
My laboratory currently concentrates on the study of cave fishes, viewing them as powerful models for the investigation of interesting problems in evolutionary biology, including speciation, the genetics of trait evolution, and regressive evolution.
About eighty species of cave fishes are presently known world-wide, and the rate of discovery of new ones is accelerating. While these species fall into a number of very distinct and different fish families, they share a set of characteristics typical of all cave animals. The most obvious of these traits are reduced eyes and pigment, but they also tend to share other heightened extraoptic senses (e.g., taste or touch) and altered metabolic rates. All of these characteristics are complex traits with multiple genetic bases. The Waterfall Climbing Cave Fish of Thailand, Cryptotora thamicola , pictured below, illustrates the reduction in eyes and pigmentation typical of cave fishes (Photo by Denis Belliveau). This is one of only three species of cave vertebrates that completely lacks eyes as an adult.
This set of characters, typical of even distantly related obligate cave dwellers (troglobites), can be termed the "troglomorphic suite." The troglomorphic suite in cave fishes exemplifies evolutionary convergence resulting from life in similar environments. It's dark in caves and, generally, there isn't much available food. Cave fishes don't need eyes and pigmentation and they are obviously better off with their other senses honed. Lower food needs on short rations is of advantage, also.
These changes raise several important questions. The first is that of regressive evolution. The evolution of some aspects of the troglomorphic suite, such as heightened extraoptic sensory abilities or a more efficient metabolism, are easily accounted for by the mechanism of natural selection. On the other hand, the evolutionary mechanisms that bring about loss of eyes or pigmentation are less apparent. It is difficult to comprehend specific and significant advantages for the loss of eyes and pigmentation in the cave environment. (This is not to say that there are none or that no good hypotheses have been advanced for these regressive changes. Simply, that the selective bases for the constructive changes are more easily apparent.)
This difficulty was recognized and discussed by Darwin in The Origin, and the mechanisms of regressive evolution remain incompletely resolved to this day. Cave fishes, classic examples of the phenomenon of regressive evolution, remain important model systems for its study.
Cave fishes are also useful for investigating the genetic changes that underly trait evolution. Partly, because reductions of eyes and pigmentation are such dramatic changes, but also because there are many cave fish species. Comparative cave fish biology introduces into evolutionary biology an element not generally found outside of experimental science: the power of replication. The ability to replicate an experiment, to do it over and over again with or without changes in important variables, gives laboratory science its power to develop and test new hypotheses. With few exceptions, replication is generally absent from evolutionary studies. One of the themes of my research is to find ways to introduce the power of replication into evolutionary biology.
The traits that are typical of cave fishes, and distinguish them from the fishes of surface waters, are the results of evolutionary histories in similar environments. Each cave fish species represents an independent evolutionary experiment. The net results for each species are similar, but the underlying genetic bases may, or may not, be. Thus, comparative cave fish biology has the potential to reveal the different pathways that evolution might take towards the same end. The variety of pathways taken in the replicated set is informative of the scope and plasticity of the evolutionary process.
Cave fishes are also useful models for the study of complex trait variation. By definition, complex traits are those that are determined by the cumulative effects of many different gene loci. For example, we know that the tendency to gain weight varies from one person to the next and this variation has a genetic basis. Given the same diet, some persons will gain weight while others will maintain or lose weight. Variability in susceptibility to weight gain among individuals is caused by differences at many different gene loci. Because many gene loci are involved with the trait, the genetic basis of weight gain is resistant to analysis. In general, when different genes can cause the same trait, it is difficult to identify them individually.
In spite of the difficulties, unraveling the genetic basis of complex traits is of great importance for practical reasons. Most of the interesting and commercially significant characteristics of domestic animals and cultivated plants are complex traits. Obvious examples include, milk production in cows, body composition of swine or poultry, disease resistance in plants, etc. In humans, the tendencies towards many diseases, such as diabetes or familial cancers, are complex traits with multiple genetic causes. Thus, complex traits have economic and medical significance.
Complex traits also have a theoretical importance for biologists, because most biological evolution involves changes in characters with complex genetic determinants. We can never truly understand biological evolution without understanding the genetics of complex traits.
At least one cave fish, the blind Mexican tetra, Astyanax mexicanus, is a good model for studying the evolution and genetics of complex traits. There are numerous blind/depigmented cave populations of this species, as well as eyed/pigmented, surface forms. All of the forms can be interbred and this facilitates gene identification and mapping. Other such cave fish models will become available for study.
Thus, our focus on cave fishes affords an opportunity to study many different current problems in evolutionary biology and to relate the results to emerging findings in molecular and developmental biology.
Graham Proudlove's checklist of all known trolobitic fishes: http://www.ccl.umist.ac.uk/staff/grahamp/pisces/list.htm
Please address correspondence to: rb4@scires.nyu.edu
Webpage and authored links © 1998, 1999, 2000 Richard Borowsky, Department of Biology, New York University.
No reproduction of content without prior consent of the author.