Freethought & Rationalism Archive

scigirl · 11-01-2002, 03:10 PM

I came across some interesting articles on the evolution of the placenta during the fetal circulatory system thread.

Here's some tidbits (I clipped out parts of the abstract for brevity). One scientist group's hypothesis as to why the placenta evolved:

<a href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=121432 80&dopt=Abstract" target="_blank">Nausea and vomiting of pregnancy: an evolutionary hypothesis</a>

Quote:

...Epidemiologic studies suggest that pregnancies accompanied by "morning sickness" have better outcomes than asymptomatic pregnancies. The intimate connection between immunogenetic identity, chemoidentity, and chemocommunication by olfactory mechanisms suggests a relationship between maternal symptoms and maternal accommodation of paternal antigens contained in the fetoplacental unit. Most mammalian species utilize olfaction to reduce inbreeding and thus do not require an intimate placental connection between mother and fetus. The evolution of Homo sapiens included prolonged periods of small, genetically homogeneous foraging groups which limited selection of genetically heterogeneous mates. Adaptation to this circumstance included a reduction of olfactory precision in mate selection and a more intimate association between mother and fetus, the hemochorial placenta.

How did the placenta evolve? Well most likely through some gene duplcations...

<a href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=118618 91&dopt=Abstract" target="_blank">Chorionic gonadotropin has a recent origin within primates and an evolutionary history of selection.</a>

Quote:

Chorionic gonadotropin (CG) is a critical signal in establishing pregnancy in humans and some other primates, but this placentally expressed hormone has not been found in other mammalian orders. The gene for one of its two subunits (CG beta subunit [CGbeta]) arose by duplication from the luteinizing hormone beta subunit gene (LHbeta), present in all mammals tested. In this study, 14 primate and related mammalian species were examined by Southern blotting and DNA sequencing to determine where in mammalian phylogeny the CGbeta gene originated...At least two subsequent duplication events occurred in the catarrhine primates, all of which possess multiple CGbeta copies. The LHbeta-CGbeta family of genes has undergone frequent gene conversion among the catarrhines, as well as periods of strong positive selection in the New World monkeys (platyrrhines). In addition, newly generated DNA sequences from the promoter of the CG alpha subunit gene indicate that platyrrhine monkeys use a different mechanism of alpha gene expression control than that found in catarrhines.

<a href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=107253 51&dopt=Abstract" target="_blank">Adaptive diversification within a large family of recently duplicated, placentally expressed genes.</a>

Quote:

The pregnancy-associated glycoproteins (PAG) are putative peptide-binding proteins and products of a large family of genes whose expression is localized to the placental surface epithelium of artiodactyl species. We have tested the hypothesis that natural selection has favored diversification of these genes by examining patterns of nucleotide substitution in a sample of 28 closely related bovine, caprine, and ovine family members that are expressed only in trophoblast binucleate cells. Three observations were made. First, in codons encoding highly variable domains of the proteins, there was a greater accumulation of both synonymous and nonsynonymous mutations than in the more conserved regions of the genes. Second, in the variable regions, the mean number of nonsynonymous nucleotide substitutions per site was significantly greater than the mean number of synonymous substitutions per site. Third, nonsynonymous changes affecting amino acid charge occurred more frequently than expected under random substitution. This unusual pattern of nucleotide substitution implies that natural selection has acted to diversify these PAG molecules at the amino acid level, which in turn suggests that these molecules have undergone functional diversification. We estimate that the binucleate cell-expressed PAG originated 52 +/- 6 million years ago, soon after the divergence of the ruminant lineage. Thus, rapid functional diversification of PAG expressed in trophoblast binucleate cells seems to have been associated with the origin of this unique placental adaptation.

...and my favorite proposed mechanism: that a virus did it:
<a href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=106938 09&dopt=Abstract" target="_blank">Syncytin is a captive retroviral envelope protein involved in human placental morphogenesis.</a>

Quote:

Many mammalian viruses have acquired genes from their hosts during their evolution. The rationale for these acquisitions is usually quite clear: the captured genes are subverted to provide a selective advantage to the virus. Here we describe the opposite situation, where a viral gene has been sequestered to serve an important function in the physiology of a mammalian host. This gene, encoding a protein that we have called syncytin, is the envelope gene of a recently identified human endogenous defective retrovirus, HERV-W. We find that the major sites of syncytin expression are placental syncytiotrophoblasts, multinucleated cells that originate from fetal trophoblasts. We show that expression of recombinant syncytin in a wide variety of cell types induces the formation of giant syncytia, and that fusion of a human trophoblastic cell line expressing endogenous syncytin can be inhibited by an anti-syncytin antiserum. Our data indicate that syncytin may mediate placental cytotrophoblast fusion in vivo, and thus may be important in human placental morphogenesis.

scigirl

ps418 · 11-03-2002, 08:22 AM

There's an interesting article on the evolution of placentas in the fish(!?) genus Poeciliopsis in the most recent issue of Science. Haven't read the article yet, but it looks interesting.

Quote:

The evolution of complex organs is a source of controversy because they require the contributions of many adaptations to function properly. We argue that placentas are complex, that they have evolved multiple times in Poeciliopsis, and that there are closely related sister taxa that have either no placentas or intermediate stages in the evolution of a placenta. Furthermore, placentas can evolve in 750,000 years or less, on the same time scale as suggested by theoretical calculations for the evolution of complex eyes. Independent origins of such complexity, accompanied by sister taxa that either lack or have intermediate stages in the evolution of the trait, present an opportunity to study the evolution of novelty and complexity from a comparative, evolutionary perspective.

Reznick, David N., Mateos, Mariana, Springer, Mark S., Independent Origins and Rapid Evolution of the Placenta in the Fish Genus Poeciliopsis. Science 2002 298: 1018-1020

abe smith · 11-04-2002, 07:07 AM

A while back (sorry to be so totally imprecise), I came across a reliable article about the (human) placenta, which said that the development of the placenta is triggered/controlled by some agent brought to the ovum >> zygote by the SPERMATOZOON = the MALE gamete. I thought that's a fascinating fact, if it is indeed a fact; and I wish I could remember whr/ I dreamt the citation, or, if it's real, where to find it. This post is not too helpful, for sure, Scigirl.... (of course it apposites to my own "King Charles's head*, that w/o a human-Father's gamete contribution, Their Lord the Chrx Saviour had to have been a *female*. >>> If the human placenta cannot occur w/o the presence/agency of the male gamete, there's another proof that that Catholic dogma (of the Virgin Conception/Birth of Jesus) is a damn lie.)

Dr.GH · 11-04-2002, 10:23 AM

The livebearers such as Embiotocidae (Surfperch) have young with hyperdeveloped dorsal and anal fins. These appear bright red while within the mother, and exchange O2 with the mother. I don't know if they also absorb nutrtients.

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11-04-2002, 07:07 AM	#3
abe smith Veteran Member Join Date: Apr 2001 Location: springfield, MA. USA Posts: 2,482	A while back (sorry to be so totally imprecise), I came across a reliable article about the (human) placenta, which said that the development of the placenta is triggered/controlled by some agent brought to the ovum >> zygote by the SPERMATOZOON = the MALE gamete. I thought that's a fascinating fact, if it is indeed a fact; and I wish I could remember whr/ I dreamt the citation, or, if it's real, where to find it. This post is not too helpful, for sure, Scigirl.... (of course it apposites to my own "King Charles's head, that w/o a human-Father's gamete contribution, Their Lord the Chrx Saviour had to have been a female*. >>> If the human placenta cannot occur w/o the presence/agency of the male gamete, there's another proof that that Catholic dogma (of the Virgin Conception/Birth of Jesus) is a damn lie.)

11-04-2002, 10:23 AM	#4
Dr.GH Veteran Member Join Date: Jul 2001 Location: Dana Point, Ca, USA Posts: 2,115	The livebearers such as Embiotocidae (Surfperch) have young with hyperdeveloped dorsal and anal fins. These appear bright red while within the mother, and exchange O2 with the mother. I don't know if they also absorb nutrtients.

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