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I’ve been much delayed, since I am starting to work seriously on my dissertation. This sucks up an amazing amount of brainpower. Initially I was planning on delaying this post another day, but have now been fortified with a BLT (heavy on the bacon) and am ready to tackle the topic. It is one that interests me because it combines two fascinating topics, malaria and sex ratios.
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.
I HAVE ALREADY mentioned one type of selfish genetic element. These are mobile elements that can move about and reproduce within the genome, and include the transposon and retrotransposons. A second similar type of selfish genetic element are the homing endonucleases. These come in two forms, as introns which are spliced out of RNA and then translated into protein instead of being discarded, or as inteins that splice out of the protein once it has been synthesized. In both cases the homing endonuclease then during meiosis attacks the allele that does not contain the homing endonuclease intron or intein and triggers DNA repair that duplicates the homing endonuclease’s sequence. Since mobile elements and homing endonucleases either attack at a wide variety of sites or duplicate onto both of a pair of chromosomes, they are passed on according to Mendelian inheritance patterns. But there are other selfish genetic elements that are passed on preferentially, and a new paper in Genetica focuses on the effects of these selfish elements upon fertility in carrier males.
I RECENTLY REVIEWED some of the advantages and disadvantages of sexual and asexual reproduction. Sexual reproduction is thought to be chiefly useful in the elimination of negative mutations, while these build up in asexually reproducing lineages. Without some type of genetic recombination it is thought that asexual reproduction will lead eventually to extinction.
ON FEBRUARY 8 my blog had its best day ever, as upwards of a dozen (!!) viewers flocked to my page on sexual and asexual reproduction. Interestingly, most of the traffic was from tag surfers tracking the tag “sex”. I have reached the obvious conclusion that to draw more traffic, I need to sex up my blog. So kick the kids out of the room and continue to the explicit!! photograph below the break.
AMONG EUKARYOTES, sexual reproduction is more common than asexual reproduction, and often organisms that reproduce asexually also have a supplementary means of sexual reproduction. While sexual reproduction does have some drawbacks, it appears that in general the benefits outweigh these.