Allow me to introduce you to a whole gigantic superclade with which many of you may not be familiar, and some other groups in the grand hierarchy of animal evolution that I’ve mentioned quite a few times before, but would like to clear the fog with some simple definitions. Consider this a brief primer in some major animal groupings. Here’s a greatly simplified cladogram; I’ve left off quite a few groups to make the story simple.
Almost ten years ago, there was a spectacular fossil discovery in China: microfossils, tiny organisms preserved by phosphatization, that revealed amazing levels of fine detail. These specimens were identified as early animal embryos on the basis of a number of properties.
There are some concerns about the interpretation, though. One troubling aspect of their distribution is that they are all only in the cleavage stage: we don’t see any gastrulas, the stage at which embryonic cells undergo shape changes and begin to move in a specific, directed manner. Studies of taphonomy (analyses of the processes that lead to fossilization) have shown that these later stages are particularly difficult to preserve, which potentially explains why we’re seeing a biased sample. Another unusual bias in the sample is that all of the embryos exhibit that regularity of division that produces equal-sized blastomeres—yet many invertebrate embryos have early asymmetric cleavages that produce recognizable, stereotyped distributions of cells. That asymmetry could be a feature that evolved late, but at the same time, some of the fossils were described as resembling molluscan trefoil embryos. Why aren’t the examples of early asymmetry translated into a later asymmetry?
Now there’s another reason to question the identity of the Doushantuo microfossils: they may be bacterial.
Look! It’s a dumbo octopus! There are some very good shots of this weird creature swimming near the middle.
Everyone knows you’re supposed to be reading Deep Sea News today for all things cephalopodial, right?
Well, if we can’t find the new Architeuthis video, we can at least enjoy a little Cephalopodmas carol, Squid and Whale.
If you’d like something more traditional, here are the lyrics for the Twelve Days of Cephalopodmas. You already know the music.
Lastly, should you really want to get into the festive spirit of the holiday, here are some photos of a whale necropsy. Warning: there is blood, there are guts. How much? Well, they used a large backhoe as a retractor.
We have a sign: there are reports
of a new video from Tsunemi Kubodera of an Architeuthis—unfortunately, I haven’t been able to find a copy of the video online anywhere yet. If anyone finds it, let me know!
There’s a small and rather grainy copy of the video on the BBC website!
The copy at CNN is of much higher quality.
Oh, man, I feel for the kids nowadays. When I was an itty-bitty dinosaur-happy tyke, it seemed like there was a manageable amount of Latin nomenclature you had to memorize to keep up with the dinosaur clan. Now it’s like there’s a new one added every week, and you’ve got to be a freakin’ genius to be able to follow them all. Kids do still go wacky over dinosaurs, right? We haven’t gone so far down the tubes that the little nerds are neglecting their paleontology, have we?
Anyway, there’s a new one out of Spain, Turiasaurus riodevensis, an old school sauropod, and it’s a big one. Pictures below the fold…
I’ve just read the article on the parthenogenetic Komodo dragons in Nature, and it’s very cool. They’ve analyzed the genetics of the eggs that have failed to develop (the remainder are expected to hatch in January) and determined that they were definitely produced without the aid of a male.
We analysed the parentage of the eggs and offspring by genetic fingerprinting. In the clutches of both females, we found that all offspring produced in the absence of males were parthenogens: the overall combined clutch genotype reconstructed that of their mother exactly. Although all offspring were homozygous at all loci, they were not identical clones. Parthenogenesis was therefore confirmed by exclusion (clutches had different alleles from potential fathers) and by the fact that the probability of obtaining a clutch of homozygous individuals after sexual reproduction was very low (P<<0.0001). Sungai’s resumption of sexual reproduction confirmed that parthenogenesis was not a fixed reproductive trait (that is, it is facultative) and that asexual reproduction is likely to occur only when necessary.
That line about “all offspring were homozygous at all loci, they were not identical clones” might need a little more explanation. Mama Dragon is heterozygous at some loci, but the meiotic mechanism that produces a diploid egg means that one cleavage (most likely the second meiotic cleavage) was suppressed, so both homologous chromosomes in the resultant ovum were derived from the same replicated DNA strand. They are not clones of the mother, because they are all homozygous while she was heterozygous; they are not identical, because which of each of the paired homologous chromosomes was passed on to an individual is random.
(I’m a little confused by the statement that they offspring are homozygous at all loci, though; that would imply that there was no crossing over at all in meiosis I, which doesn’t sound right. There ought to be reduced heterozygosity but not complete homozygosity, unless reptiles are weirder than I thought.)
The other useful snippet of information is that sex determination in these reptiles is of the WW/WZ type, where the females are the heterogametic sex. Since all of the progeny of parthenogenesis are homozygous, they are all of the homogametic genotype, and therefore male.
Parthenogenesis can also bias the sex ratio: in Varanus species, females have dissimilar chromosomes (Z and W), whereas the combination ZZ produces males10, so the parthenogenetic mechanism can produce only homozygous (ZZ or WW) individuals and therefore no females.
This has theological implications, obviously. We can now understand how a female could give rise to a male by parthenogenesis: Mary Mother of God must have been a heterogametic reptoid. David Icke will be so pleased.
Watts PC, Buley KR, Sanderson S, Boardman W, Ciofi C, Gibson R (2006) Parthenogenesis in Komodo dragons. Nature 444:1021-1022.
