Tuesday, January 16, 2007

Distinguishing Design from Accidental Events: Part Two

Uncommon Descent featured this post debunking some Talk Origins concepts. A continuation of comments on the UD post- quoting Shauer’s Article:

"They [transposons] are Necessary for Embryonic Development

The research, published in the October issue of Developmental Cell, suggests that retrotransposons may not be just the “junk DNA” once thought, but rather appear to be a large repository of start sites for initiating gene expression. Therefore, more than one third of the mouse and human genomes, previously thought to be nonfunctional, may play some role in the regulation of gene expression and promotion of genetic diversity. Dr. Barbara B. Knowles and colleagues from The Jackson Laboratory in Bar Harbor, Maine, found that distinct retrotransposon types are unexpectedly active in mouse eggs, and others are activated in early embryos. Surprisingly, by acting as alternative promoters, retrotransposon-derived controlling elements drive the coordinated expression of multiple mouse genes. The researchers think that expression of retrotransposons during very early stages may contribute to the reprogramming of the mammalian embryonic genome, a prerequisite for normal development."


Indeed, endogenous retroviruses can be connected with function, specific ones involving the developmental stages of mammals. "Syncytins, products of the env gene of HERV-W and HERV-FRD, contribute to human placenta development. Similar genes are also found in mouse and sheep. Indeed, the sheep ERV genes have been shown essential for sheep reproduction. Furthermore, regulation of the human syncytin-1 gene involves a complex regulation network including both viral and host factors."1

In addition, Endogenous retroviruses regulate periimplantation placental growth and differentiation reports the following:

"Endogenous retroviruses (ERVs) are fixed and abundant in the genomes of vertebrates. Circumstantial evidence suggests that ERVs play a role in mammalian reproduction, particularly placental morphogenesis, because intact ERV envelope genes were found to be expressed in the syncytiotrophoblasts of human and mouse placenta and to elicit fusion of cells in vitro. We report here in vivo and in vitro experiments finding that the envelope of a particular class of ERVs of sheep, endogenous Jaagsiekte sheep retroviruses (enJSRVs), regulates trophectoderm growth and differentiation in the periimplantation conceptus (embryo/fetus and associated extraembryonic membranes). The enJSRV envelope gene is expressed in the trophectoderm of the elongating ovine conceptus after day 12 of pregnancy. Loss-of-function experiments were conducted in utero by injecting morpholino antisense oligonucleotides on day 8 of pregnancy that blocked enJSRV envelope protein production in the conceptus trophectoderm. This approach retarded trophectoderm outgrowth during conceptus elongation and inhibited trophoblast giant binucleate cell differentiation as observed on day 16. Pregnancy loss was observed by day 20 in sheep receiving morpholino antisense oligonucleotides. In vitro inhibition of the enJSRV envelope reduced the proliferation of mononuclear trophectoderm cells isolated from day 15 conceptuses. Consequently, these results demonstrate that the enJSRV envelope regulates trophectoderm growth and differentiation in the periimplantation ovine conceptus. This work supports the hypothesis that ERVs play fundamental roles in placental morphogenesis and mammalian reproduction."2

If transposons insert fairly randomly as TalkOrigins claims, then that which preceeded a random insertion affecting embryonic development is most interesting. How did sheep reproduce prior to this putative random event? Was it a random event?


References:

1. 'Were Retroviruses Created Good?'; by Dr. Yingguang Liu; http://www.answersingenesis.org/articles/am/v1/n2/were-retroviruses-created-good

2. 'Endogenous retroviruses regulate periimplantation placental growth and differentiation'; PNAS | September 26, 2006 | vol. 103 | no. 39 | 14390-14395; Kathrin A. Dunlap, Massimo Palmarini, Mariana Varela, Robert C. Burghardt, Kanako Hayashi, Jennifer L. Farmer, and Thomas E. Spencer

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