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| How Do Genomes Produce Two Very Different Forms: Males And Females? |
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| SciMed - Genetics & Genome | |||
| TS-Si News Service | |||
| Sunday, 06 January 2008 20:00 | |||
TS-Si  Genetics & The GenomeNew York, NY, USA. Ninety percent of every mammal's genome is dark matter harboring ancient viral DNA that infected our ancestors from as far back as the age of the dinosaurs.
Scientists have uncovered clues as to how our ge...Worcester, MA, USA. A chromosome layer folds into contiguous yarns that harbor groups of genes and regulatory elements, enabling contact for coordinated development work.
This is a new layer in the complex organization of ch...The differences between males and females consist of traits related to mating (including weapons used when competing for mates) and display traits used to seduce them.
So, how do genomes produce two such very different forms – males and females?
 "Differences between males and females make up a substantial component of diversity in the biological world, with the sexes often differing in size, shape and color," says Dr. Steve Chenoweth, a Senior Lecturer with the School of Integrative Biology at the University of Queensland. Chenoweth says "The catch from a genetic standpoint is that males and females share almost all of their genes. Because of this, many genes that benefit one sex may actually be harmful to the other."
Genetic Constraints and the
Evolution of Display Trait Sexual Dimorphism by Natural and Sexual Selection. Stephen F. Chenoweth, Howard D. Rundle, and Mark W. Blows. American Naturalist 171:22–34. 0003-0147/2008/17101-42328. DOI: 10.1086/523946. In biology, an organism's genome is the hereditary information encoded in the DNA, including both genes and non-coding sequences. The human genome is predominantly shared between the two sexes and imposes contraints that affect the degree to which sexual dimorphism evolves, reflecting a historical interaction between sexually antagonistic selection.
"In birds, a gene that causes brightly coloured plumage in males may have advantages in terms of attracting a mate whereas its effect in a female could be a distinct disadvantage, making her more noticeable to predators for example. These so-called sexually antagonistic genes are a real problem, and how species have come to deal with their detrimental effects while maintaining their benefits remains a mystery for modern genetics," says Chenowath.
These questions are of special interest to people with a history of Harry Benjamin Syndrome (HBS). The mechanisms for achieving sexual dimorphism, however mysterious, do work in almost all cases, but HBS is an obvious exception. Understanding the standard situation will aid other investigations into the origins of HBS.
Sexual conflict may lead to sexually antagonistic co-evolution, in which one sex (usually males) evolves a "manipulative" trait which is countered by a "resistance" trait in the other sex. This kind of sexually antagonistic selection on shared traits generates intralocus sexual conflict in which one sex impedes the adaptive evolution of the other sex. Some regard sexual conflict as a subset of sexual selection, while others point to accumulating evidence that suggests it is a separate evolutionary phenomena.
The evolution of sexual dimorphism is the ultimate means available to resolve these conflicts. There have been now classic theoretical treatments of the subject and a great deal of interest by scientists. However, substantial empirical work remains to gain a comprehensive understanding of the evolution of sexual dimorphism. Especially interesting is the identification of the sources of sexually antagonistic selection and how genetic constraints between the sexes are overcome.
The conflicting interests of males and females in reproduction are thought to be a key source of sex-specific selection on such traits. The evolution of sexual dimorphism is hampered, however, because the sexes share the majority of their genomes. An adaptive response to selection in one sex may therefore cause nonadaptive changes in the other. This process shows a high degree of correlation.
Dr. Chenoweth says that while the genes for sex differences may be shared, the trigger for their development or suppression might be located on sex chromosomes. "Once we understand where these sex-specific triggers are and how they work, there is potential for developing intervention methods to control sex differences in the development of certain diseases such as heart disease — much higher rate among men — as well as areas such as longevity — women on average live a lot longer than men," he said.
A recent study addresses the combined issues of genetic control and sex-specific selection. Steve Chenoweth and colleagues, Howard Rundle and Mark Blows, from the University of Queensland, conducted a comprehensive investigation of the evolution of male-female differences in a set of sexual display traits in the Australian fruit fly, Drosophila serrata.
The researchers used a series of natural populations collected across a wide area along the east coast of North Queensland's tropical rainforests. The flies are so small — just 2mm long — they must be netted then sucked up through a plastic straw by researchers before being transferred to glass bottles for transportation back to Dr Chenoweth's laboratory.
Both male and female Drosophila serrata use a set of pheromonal displays during mating, but differ in the relative concentrations of the individual compounds. The authors show that the amount of sexual dimorphism varies, and that these differences among populations are genetic.
The researchers used classic quantitative genetic methods to demonstrate that the relaxation of genetic constraints appears to have occurred largely via genes on the X chromosome. “There are a number of mechanisms by which genetic constraints can be overcome,” says Dr. Steve Chenoweth, “Our results suggest that sex-linkage may be key.” By manipulating both natural and sexual selection and observing the evolutionary response in the laboratory, the authors also show that, as expected, sexual selection appears to generate sex-specific optima for these pheromones. Contrary to expectation, however, females and not males responded to sexual selection.
“This suggests that the classic scenario of males as the sole target of sexual selection may be overly simplistic,” states Dr. Howard Rundle. “If you want to know why males and females differ, you need to consider all forms of selection on both sexes.”
Genetic Constraints and the Evolution of Display Trait Sexual Dimorphism by Natural and Sexual Selection. Stephen F. Chenoweth, Howard D. Rundle, and Mark W. Blows. American Naturalist 171:22–34. 0003-0147/2008/17101-42328. DOI: 10.1086/523946.
Abstract. The evolution of sexual dimorphism involves an interaction between sex‐specific selection and a breakdown of genetic constraints that arise because the two sexes share a genome. We examined genetic constraints and the effect of sex‐specific selection on a suite of sexually dimorphic display traits in Drosophila serrata. Sexual dimorphism varied among nine natural populations covering a substantial portion of the species range. Quantitative genetic analyses showed that intersexual genetic correlations were high because of autosomal genetic variance but that the inclusion of X‐linked effects reduced genetic correlations substantially, indicating that sex linkage may be an important mechanism by which intersexual genetic constraints are reduced in this species. We then explored the potential for both natural and sexual selection to influence these traits, using a 12‐generation laboratory experiment in which we altered the opportunities for each process as flies adapted to a novel environment. Sexual dimorphism evolved, with natural selection reducing sexual dimorphism, whereas sexual selection tended to increase it overall. To this extent, our results are consistent with the hypothesis that sexual selection favors evolutionary divergence of the sexes. However, sex‐specific responses to natural and sexual selection contrasted with the classic model because sexual selection affected females rather than males. [ Full Text (PDF) ]  The TS-Si News Service is a collaborative effort by TS-Si.org editors, contributors, and corresponding institutions. Sources can include the cited individuals and organizations, as well as TS-Si.org staff contributions. Articles and news reports do not necessarily convey official positions of TS-Si, its partners, or affiliates. We welcome your comments. Use the form below to leave a public comment or send private correspondence via the TS-Si Contact Page. We will not divulge any personal details or place you on a mailing list without your permission.TS-Si is dedicated to the acceptance, medical treatment, and legal protection of individuals correcting the misalignment of their brains and their anatomical sex, while supporting their transition into society as hormonally reconstituted and surgically corrected citizens.
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