[Paleopsych] On Chromosomes and Sex Determination

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Evolutionary Biology: On Chromosomes and Sex Determination

    The following points are made by Brian Charlesworth (Current Biology
    2004 14:R745):
    1) Determination of sexual identity by genes associated with highly
    differentiated sex chromosomes is often assumed to be the norm, given
    our familiarity with the X and Y chromosomes of mammals and model
    organisms such as Drosophila. Even within tetrapod vertebrates,
    however, there is a wide diversity of sex determination mechanisms,
    with many examples of species with genetic sex determination but
    microscopically similar X and Y chromosomes, and numerous cases of
    environmental sex determination [1]. There is an even wider range of
    sexual systems in teleost fishes, with examples of self-fertilizing
    hermaphrodites [2], sequential hermaphrodites [3], and environmental
    sex determination [1].
    2) Even where sex is genetically determined, the mechanisms vary
    enormously, with clearly distinguishable sex chromosomes being very
    rare [1,4]. It is easiest to see the footprints of the evolutionary
    forces that drive the evolution of sex chromosomes in cases where the
    divergence of X and Y chromosomes has not reached its limit, with no
    genetic recombination between X and Y chromosomes over most of their
    length and a lack of functional genes on the Y chromosome [5]. The
    comparative genetics of sex determination systems in fish species may
    thus yield important insights into the evolution of sex chromosomes.
    3) Despite pioneering classical genetic studies of sex determination
    in fish such as the medaka, the guppy, and the platyfish [1], it has
    been difficult to obtain detailed genetic information on sex
    chromosome organisation in these species. With modern genomic methods,
    however, it is now feasible, but laborious, to characterize the sex
    determining regions of fish genomes. Studies of chromosomal regions
    that determine male development in two unrelated groups of fish
    species show the promise of this approach.
    References (abridged):
    1. Bull, J.J. (1983). Evolution of Sex Determining Mechanisms. (Menlo
    Park, CA: Benjamin Cummings)
    2. Weibel, A.C., Dowling, T.E. and Turner, B.J. (1999). Evidence that
    an outcrossing population is a derived lineage in a hermaphroditic
    fish (Rivulus marmoratus). Evolution 53, 1217-1225
    3. Charnov, E.L. (1982). The Theory of Sex Allocation. (Princeton, NJ:
    Princeton University Press)
    4. Volff, J.-N. and Schartl, M. (2001). Variability of sex
    determination in poeciliid fishes. Genetica 111, 101-110
    5. Charlesworth, B. and Charlesworth, D. (2000). The degeneration of Y
    chromosomes. Phil. Trans. Roy. Soc. Lond. B. 355, 1563-1572
    Current Biology http://www.current-biology.com
    Related Material:
    The following points are made by Deborah Charlesworth (Current Biology
    2004 14:R271):
    1) Plant sex chromosomes are particularly interesting because they
    evolved much more recently than those of mammals or Drosophila -- most
    plants with separate sexes seem to have evolved recently from
    ancestors with both sex functions [1,2]. Plant sex chromosomes may
    thus tell us about the initial stages of the evolutionary process that
    has led to the massive gene loss that has occurred in Y chromosomes.
    2) The sex determination system of papaya (Carica papaya) has been
    studied genetically since 1938, when it was established that an
    apparently single locus determines the male, female or hermaphrodite
    state. As in many familiar animal systems, including Drosophila and
    mammals, female papaya are the homozygous sex, while males and
    hermaphrodites are heterozygotes. In most dioecious plants -- those
    with separate sexes, rather than hermaphroditism -- males are also the
    heterozygous sex [1].
    3) Many animals and most dioecious plant species, such as Silene
    latifolia, have a visibly distinctive X/Y sex chromosome pair. The
    mammalian Y is smaller than the X, whereas the S. latifolia Y
    chromosome is larger than its X. Many dioecious plants, however,
    including papaya and kiwi fruit [3], have no such chromosome
    heteromorphism; in these species, the sex-determining genes seem to
    map to small regions of one normal-looking chromosome [3,4].
    4) To understand the papaya sex determining region, a detailed map has
    now been made of the papaya chromosome (chromosome LG1) carrying the
    sex-determining genes [5]. At present, most of the markers used are
    "anonymous" DNA sequence variants, not in coding sequences, and
    detected by the "amplified fragment length polymorphism" (AFLP)
    approach. As expected for a chromosome carrying the sex-determining
    genes, LG1 includes markers that co-segregate perfectly with sex. The
    finding of many such markers --225 out of 342 LG1 markers -- indicates
    that the sex-determining genes are spread over an extensive region
    that could include many genes. Physical mapping of the non-recombining
    genome region (obtained by sequencing bacterial artificial chromosome
    (BAC) clones carrying sequences corresponding to some of the markers)
    allowed Liu et al.[5] to estimate that the region involved in sex
    determination in papaya extends over roughly 4.4 Mb, only about 10% of
    chromosome LG1.
    References (abridged):
    1. Westergaard, M. (1958). The mechanism of sex determination in
    dioecious plants. Adv. Genet. 9, 217-281
    2. Charlesworth, B. and Charlesworth, D. (1978). A model for the
    evolution of dioecy and gynodioecy. Am. Nat. 112, 975-997
    3. Harvey, C.F., Gill, C.P., Fraser, L.G., and McNeilage, M.A. (1997).
    Sex determination in Actinidia. 1. Sex-linked markers and progeny sex
    ratio in diploid A. chinensis. Sex. Plant Repro. 10, 149-154
    4. Semerikov, V., Lagercrantz, U., Tsarouhas, V., Ronnberg-Wastljung,
    A., Alstrom-Rapaport, C., and Lascoux, M. (2002). Genetic mapping of
    sex-linked markers in Salix viminalis L. Heredity 91, 293-299
    5. Liu, Z., Moore, P.H., Ma, H., Ackerman, C.M., Ragiba, M., Pearl,
    H.M., Kim, M.S., Charlton, J.W., Yu, Q., and Stiles, J.I. et al.
    (2004). A primitive Y chromosome in Papaya marks the beginning of sex
    chromosome evolution. Nature 427, 348-352
    Current Biology http://www.current-biology.com
    Related Material:
    The following points are made by Richard E. Lenski (Science 2001
    1) Why have some organisms evolved the capacity for sexual
    reproduction, whereas others make do with reproducing asexually? Since
    the time of August F. Weismann (1834-1914), most biologists have been
    taught that sex produces variation and thereby promotes evolutionary
    adaptation. But how does sex achieve this effect, and under what
    circumstances is it worthwhile?
    2) The traditional explanation for sex is that it accelerates
    adaptation by allowing two or more beneficial mutations that have
    appeared in different individuals to recombine within the same
    individual. Without sexual recombination, individual clones that
    possess different beneficial mutations compete with one another,
    slowing adaptation by clonal interference. Sex, according to the
    traditional explanation, allows simultaneous improvements at several
    genetic loci, whereas multiple adaptations must occur sequentially in
    clonal organisms.
    3) The above explanation, however, has recently come into question.
    First, sex imposes a 50 percent reduction in reproductive output: If a
    female can produce viable offspring on her own, why dilute her genetic
    contribution to subsequent generations by mating with a male? Second,
    the circumstances under which this kind of model provides sufficient
    advantage to offset the cost of sex are restrictive, requiring certain
    forms of selection and environmental fluctuations. Third, alternative
    models propose that the advantage of sex lies in eliminating
    deleterious mutations rather than in combining beneficial mutations.
    Still another hypothesis, involves an interplay between deleterious
    and beneficial mutations. Finally, empirical tests of these hypotheses
    have so far failed to produce a clear winner, so the field is ripe for
    significant experiments.
    Science http://www.sciencemag.org

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