Re: chromosome number, from "Design up to Scratch?"

From: bivalve (bivalve@mail.davidson.alumlink.com)
Date: Tue Apr 29 2003 - 18:46:49 EDT

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    >>As parthenogenetic polyploid hybrids, they do not have to sort the chromosomes for meiosis."<<
    >Can you explain your terminology here, I am not familiar with parthenogenetic polyploid, and how that relates to chromosome segregation in meiosis.<

    I omitted a comma between parthenogenetic and polyploid. The parthenogenesis and the polyploidy are not especially related, except that the polyploidy apparently allows the parthenogenesis to occur. Reproduction is clonal, with eggs formed by mitosis rather than meiosis. Thus, no pairing of chromosomes in meiosis is necessary.

    I have forgotten whether this was in sphaeriids or in Lasaea, two groups of bivalves with hybrid polyploids that reproduce parthenogenetically. In either case, Diarmaid O'Foighill would be one of the authors, if it is published yet (I remember it from a meeting a few years ago). The phenomenon of hybrid polyploids that reproduce parthenogenetically is also known in several other taxa, including reptiles, fish, and snails.

    >Naturally advantageous changes in chromosomes among mammals is difficult to imagine, especially in terms of reproduction.<

    As fas as I know, the variations in chromosome number in mammals are thought to be random variants that happened to succeed via genetic drift, rather than advantageous. Thus, groups that frequently have small, isolated populations might be more prone to such changes. It is also possible that some taxa are better able to deal with such changes than are others. For example, I think I remember that a particular gene had been identified in wheat that enables it to sort out the three sets of chromosomes properly during meiosis. If this gene is not working or absent, the equivalent chromosomes from the three ancestral lineages tend to get mixed up, and meiosis fails.

        Dr. David Campbell
        Old Seashells
        University of Alabama
        Biodiversity & Systematics
        Dept. Biological Sciences
        Box 870345
        Tuscaloosa, AL 35487-0345 USA
        bivalve@mail.davidson.alumlink.com

    That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa

                     

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