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ANIMAL MODELS are widely used in medical research, sometimes in testing new drugs for safety before human trials, other times as model systems for human diseases. Like all mammals, humans and mice share most of their genes, and maintain high sequence similarity. These factors suggest that many of these genes should share the same role. A new study in Proceedings of that National Academy of Sciences examines this hypothesis.
I HAD PLANNED on covering the article on the platypus genome that came out in Nature last week, but since then this paper has been discussed in detail on Pharyngula and Adaptive Complexity and I think further discussion would be moot. I did notice while reading the paper that the unfortunate description of certain platypus genes as “reptilian” cropped up frequently. Although the authors noted that the sauropsids and synapsids are amniotes, they never mentioned that platypus genes shared with reptiles are actually basal amniote genes. Although their phylogenetic tree shows synapsids and sauropsids clearly diverging from a common amniote ancestor, they do not seem to realize referring to these ancestral amniote genes as “reptilian” suggests evolution of the platypus (and thus all synapsids) from reptiles instead of from a non-reptilian amniote.
However, today I want to talk about the platypus’ sex chromosomes. Platypuses, like the therians, have genetic sex determination. They have an XX/XY system in which males (XY) are the heterogametic sex. Many reptiles have environmental sex determination, with sex determined by factors such as incubation temperature during embryonic development. However, some reptiles and all birds have a ZW/ZZ sex determination system, with females (ZW) as the heterogametic sex.
THE MICROVIRUSES are positive-strand DNA viruses with very small genomes, typified by ΦX174 with 5,400 base pairs and nine genes. Cramming this many genes into that short a sequence requires overlapping reading frames, with gene B contained inside gene A, and gene E contained inside gene D.1 These nested genes are frame-shifted compared to the gene that contains them.
I COVERED the volvocine algae recently and promised a post on some of the genes involved in the evolution of multicellularity in this group. We still know relatively little about the genomes of volvocine algae, but research has picked out three genes involved in the evolution of multicellularity. These are invA, glsA, and regA.
MOST OF OUR study of gene networks has been done by comparison of related species to reconstruct network evolution and by knocking out specific genes to determine what the effects of their absence are. In a new paper Isalan and coworkers try something new, reprogramming genetic networks in Escherichia coli and examining the mutants to detect viability and any possible benefits to genetic pathway modification.
LOTS OF GOOD stuff appearing in the blogosphere this past week. PZ Myers has a post on how chromosome counts change and can contribute to speciation. Emile writes about beetles that chemically enslave ants. On the paleontological side of things Darren Naish wraps up his series on British cat species and Brian Switek reports on a new study showing that elephants are descended from aquatic ancestors, with links to blog coverage at other sites.
I will post tomorrow about the results of an experimental addition of links to gene networks in bacteria and later this week a survey of some of the genes involved in developing multicellularity in volvocine algae.
IN MANY CASES even large phenotypic changes can occur without much genetic change. However, occasionally a species will be placed in a situation in which its ancestor’s genes are insufficient, and if the species possesses a gene that can be co-opted into a new role, selection can favor evolution of new genes and a corresponding radiation of species. The Pieridae family of butterflies lay their eggs on plants in the Brassicaceae family, which contains mustards and cabbages. These plants have evolved to produce compounds that are harmless in undamaged leaves, but when a leaf is damaged are converted to a potent insecticide. The pierids evolved a deactivating protein that diverts the chemical reaction to produce nontoxic products. This gene evolved shortly after the plants themselves, and would have allowed these butterflies’ larvae to feed upon these plants with little competition.
THE VERTEBRATE coagulation system is a complicated cascade of enzymes, yet it evolved by the gradual addition of enzymes. It is thought that this complex system evolved by the repeated duplication and divergence of two ancestral genes. We are most familiar with the prothrombin activators as essential clotting cascade elements, yet some snakes have weaponized these enzymes.
THIS WEEK I’ve been covering some interesting instances of new gene evolution. The one I’m covering today is hard to boil down into a short title. This is a case of a new gene in hominoids as the result of retrotransposition of an aberrant mRNA transcript. Transposons showed up in the last post as well, but here they play a different role. That example involved a class II transposon, a segment of DNA that can jump around the genome. This case involves a class I transposon, a retrotransposon that transcribes itself into RNA, then copies that transcript back into DNA, and inserts it elsewhere in the genome. Here the retrotransposon accidentally retrotransposed a gene transcript instead of a retrotransposon transcript. This is not an especially rare event, but this case is unusual because the transcript itself is unusual.