godslanguage wrote:For starters:
Trichoplax adhaerens barely qualifies as an animal. About 1 millimeter long and covered with cilia, this flat marine organism lacks a stomach, muscles, nerves, and gonads, even a head. It glides along like an amoeba, its lower layer of cells releasing enzymes that digest algae beneath its ever-changing body, and it reproduces by splitting or budding off progeny. Yet this animal's genome looks surprisingly like ours, says Daniel Rokhsar, an evolutionary biologist at the University of California, Berkeley (UCB) and the U.S. Department of Energy Joint Genome Institute in Walnut Creek, California. Its 98 million DNA base pairs include many of the genes responsible for guiding the development of other animals' complex shapes and organs, he and his colleagues report in the 21 August issue of Nature.
Biologists had once assumed that complicated body plans and complex genomes went hand in hand. But T. adhaerens's genome, following on the heels of the discovery of a similarly sophisticated genome in a sea anemone (Science, 6 July 2007, p. 86), “highlights a disconnect between molecular and morphological complexity,” says Mark Martindale, an experimental embryologist at the University of Hawaii, Honolulu. Adds Casey Dunn, an evolutionary biologist at Brown University, “It is now completely clear that genomic complexity was present very early on” in animal evolution.
Science 22 August 2008:
Vol. 321. no. 5892, pp. 1028 - 1029
DOI: 10.1126/science.321.5892.1028b
GENOMICS: 'Simple' Animal's Genome Proves Unexpectedly Complex
There are three possibilities here:
1.Trichoplax is secondarily simple. In other words, its ancestors possessed a more complex body plan, but these features were lost in the lineage which gave rise to Trichoplax. The developmental genes however were retained, in either functional or nonfunctional form.
2.Trichoplax has an as yet undiscovered part of its life cycle. The developmental genes are specific to a specific part of the life cycle.
3.The developmental genes exist for no reason, and do not provide any selective advantage to the host organism.
Let's go through these possibilities one by one.
1.This would not be unprecedented. Blind cave fish possess non-functional genes which would normally code for eyes. These fish are descendants of fish which possessed functional eyes. At some point in their evolution, blind cave fish lost the ability to develop eyes, but they have retained the genes to do so.
Something similar may be going on with Trichoplax. But in my opinion, it seems as if the amount of structures that would need to be lost is simply too much. However, nature continues to surprise me, so you never know.
2. The second possibility is more interesting. Many organisms have life cycles in which juveniles and adults are radically different. Frogs and tadpoles come to mind. Frogs possess genes for making fins and gills, but these structures are absent in the adult form.
Trichoplax has never been studied in the wild, and very little is known about its native environment. It is entirely possible that the relatively simple body plan is only one part of the organism's life cycle. Presumably then, the developmental genes found in the organism's genome are needed by the other stages of the life cycle.
Is there any evidence to support this hypothesis? Yes. Researchers have found that Trichoplax is capable of sexual reproduction- analysis of the genome indicates genetic recombination occurs on occasion. However, the known form of Trichoplax lacks the means to reproduce sexually. In fact, it has only been observed to reproduce asexually. Maybe, an as yet undiscovered form of Trichoplax possesses a more complex body plan and is capable of sexual reproduction.
3. If I'm not mistaken, you are advocating the third hypothesis: the developmental genes are useless to Trichoplax. If this is the case, then we can make a simple prediction and test it against the evidence. If the genes really are useless, then mutations within them will not affect the organism. Thus, mutations will accumulate at a predictable rate. This rate will be equivalent, or nearly so, to the background mutation rate of the organism. Is this the case? In the article you cited, did the researchers find that the developmental genes were all broken? What was the ratio of synonymous to non-synonymous mutations?