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ScienceWeek
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
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EVOLUTIONARY BIOLOGY: EVOLUTION OF PLANT SEX CHROMOSOMES
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
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ON EVOLUTION AND SEXUAL REPRODUCTION
The following points are made by Richard E. Lenski (Science 2001 294:533):
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.
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