10 April 2008

Return of the bdelloid rotifer Macrotrachela sonorensis

The first animal species that I ever described was a bdelloid rotifer, Macrotrachela sonorensis, that I had found in a Mexican desert during a vacation (Örstan, 1995). As years went by, I started having doubts about the validity of that species and started questioning my judgment and desire to rush to print with what may have been inadequate sampling. In fact, Segers (2007) in his annotated checklist of rotifers included Macrotrachela sonorensis as a “species inquirenda”.

I can now rest assured, however, for Bill Birky at the University of Arizona reports on his lab page, with photographs, the rediscovery of Macrotrachela sonorensis also from a desert area, but this time in Arizona.

The indomitable bdelloid rotifers have been on this blog before (here and here). In today’s edition of Nature, there is a news article about David Mark Welch of the Marine Biological Laboratory in Woods Hole and the research he has been doing with bdelloids. Bdelloids appear to have evolved highly efficient mechanisms to repair their DNA, which is likely to get damaged during frequent episodes of desiccation most species are exposed to in their ephemeral aquatic habitats. The organization of the bdelloid genome is tetraploid, that is, instead of the more usual one pair of chromosomes, there are 2 pairs of similar chromosomes. According to David, bdelloids’ extra chromosomes may explain how they can repair their DNA efficiently and thus survive extreme desiccation in the wild and high doses of radiation in the laboratory that would turn most other creatures into heaps of dust.

The distribution of Macrotrachela sonorensis seems to be restricted to deserts, although it’s hard to be sure with only 2 records so far. This, of course, leads to the question of which came first. Did the bdelloids evolve their tetraploid genome and the associated DNA repair mechanisms before they became adapted to life in ephemeral habitats or did the repair mechanisms originate after they started living under precarious conditions? I suspect repair mechanisms and various other physiological and morphological adaptations that enable them to survive in the absence of water, such as their ability to contract their bodies into little balls, coevolved in perhaps slowly evaporating coastal brackish water ponds.

I’d better stop before speculation gets out of control.

Segers, H. 2007. Annotated checklist of the rotifers (Phylum Rotifera), with notes on nomenclature, taxonomy and distribution. Zootaxa 1564: 1-104. pdf

Örstan, A. 1995. A new species of bdelloid rotifer from Sonora, Mexico. Southwestern Naturalist 40: 255-258. (One of these days I will scan this paper and put it up on the Internet.)

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