VMartin
Posts: 525 Joined: Nov. 2006
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Jeanot July 1st:
Quote | It's clear that sexual reproduction uses different mechanisms in distantly related vertebrates, however, it doesn't necessary imply that sexual reproduction appeared independently from a pre-sexual ancestor. One could easily imagine that a lineage can evolve different sexual mechanisms (sexual chromosomes, environmental determinism) without reverting to asexuality between those. For instance, opisthoconta (animals and fungi) share common structures in their spermatozoids, and I'm not even considering these homologies within vertebrates. So it seems that sexual reproduction has a single origin in this phylum.
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Anyway facts mentioned by Davison are interesting. Germ cells in birds and mammals seem to have their origin in different embryonal structures which serves as source of different body organs. Davison:
Quote | In birds the cells destined to become the germ cells first appear in the extra-embryonic endoderm (germinal crescent) anterior to the head of the developing embryo. Incidentally, this region has no homologue in the hatched bird as the extra-embryonic endoderm is, by definition, resorbed as nutrient for the developing chick. From there the presumptive germ cells enter the circulatory system and, after a period of time in the bloodstream, penetrate the walls of the venous circulation and invade the gonad where they differentiate into the definitive gametes. In mammals the presumptive germ cells first appear in the endoderm of the allantois, a structure destined to become the urinary bladder of the adult. From here they migrate in amoeboid fashion anteriorly and laterally to reach the gonad where they complete their differentiation. Thus, there is no way that the reproductive cells of mammals can be homologized with those of birds as they originate from opposite ends of the embryonic axis and reach the gonads by completely different means.
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I would say his conclusion is more persuasive as hypothetical "One could easily imagine...". Why should I imagine something where facts do not support common ancestors having both sex? Only to prove plausibility of darwininian explanation? I do not see there any reason (inside eggs or embryo and his development) that should reorganize the source of germ cells or their journey into gonads.
-------------- I could not answer, but should maintain my ground.-
Charles Darwin
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