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Humans and chimpanzees do not look much alike on the outside. However their genome differs only by an estimated 1.3 percent. Geneticists have known for quite a while that this is not definitive.
The crucial differences between the species lie in the translation of the genetic make-up in protein building.
The Primate order includes over 375 species. The family Hominidae includes the great apes, of which humans are a representative. There are others: two species of chimpanzees (Pan troglodytes and P. paniscus), two of gorillas (Gorilla gorilla and G. beringei) and the two species of orangutans (Pongo pygmaeus and P. abelii).
Researchers at the Max Planck Institute for Evolutionary Anthropology examined the activity of around 21,000 genes in the heart, liver, kidneys, testicles and brain of both species. With regard to genetic structure and activity, the brain shows the smallest differences between humans and chimpanzees. This is true even though we differ predominantly from chimpanzees through brain functions such as speech and memory.
Because chimpanzees are our closest living relatives, the chimp genome is the most useful key to understanding human biology and evolution, next to the human genome itself.
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Uppsala, Sweden. New research identifies hundreds of biological differences between the sexes when it comes to gene expression in the cerebral cortex of humans and other primates. New findings show that some of these differences arose a very long time ago and have been preserved throughout primate evolution.
There are fundamental questions for neuroscience regarding the relative contribution of genetics versus environment to behavioral differences between the sexes. Examples of the physical, and more obvious, gender [N1] differences include average body size and weight, and genitalia design.
An Evolutionarily Conserved Sexual Signature in the Primate Brain. Björn Reinius, Peter Saetre, Jennifer A. Leonard, Ran Blekhman, Roxana Merino-Martinez, Yoav Gilad, Elena Jazin. (2008). PLoS Genetics 4(6): e1000100. doi: 10.1371 / journal.pgen.1000100 [ Download PDF ]
A team of researchers from Sweden and the United States. Uppsala Universitet, Karolinska Institutet, and The University of Chicago conducted a high precision inventory of the differences in gene expression between the sexes [N2]. This particular study did not determine functional significance of the differences, but establishes an informative baseline for further investigation. The findings appear in PLoS Genetics.
But could the differences between sexes constitute a signature of sex differences in the brain? "Knowledge about gender differences is important for many reasons. For example, this information may be used in the future to calculate medical dosages, as well as for other treatments of diseases or damage to the brain," says Elena Jazin, Associate Professor, Uppsala Universitet.
Lead author Björn Reinius notes that the study does not determine whether these differences in gene expression are in any way functionally significant.
Their particular study focuses on gene expression within the cerebral cortex — that area of the brain that is involved in such complex functions in humans and other primates as memory, attentiveness, thought processes, and language.
The team hypothesized that some of the conserved differences between the sexes might be partially explained by sexually dependent gene expression differences in the brain.
They also speculated that if differences in gene expression between males and females are functionally important, they may be conserved in the evolution of primates.
To test their hypotheses, the team measured gene expression by extracting total RNA from the occipital cortex of four males and four females in each of three primate species:
• human:Homo sapiens
• macaque:Macaca fascicularis
• marmoset:Callithrix jacchus
Transcribed cDNA (n = 24) was hybridized to human cDNA microarrays (n = 24) in male-female pairs within each species.
The yellow circles represent RNA samples from females (F) and males (M).
Arrows symbolize microarray hybridizations.
The tip of the arrow indicates that the sample was labeled with Cy5 dye and the base of the arrow that the sample was labeled with Cy3 dye.
Illustration adapted from Figure 1 of the referenced research article.
To measure the activity of specific genes, the products of genes (RNA) obtained from the brain of each animal were hybridized to microarrays containing thousands of DNA clones coding for thousands of genes.
The authors also investigated DNA sequence differences among primates for genes showing different levels of expression between the sexes. Their overall results point to:
A conserved signature of sexual gene expression dimorphism in the brains of primates.
Genes with conserved sexual gene expression dimorphism in the brain also evolve under more evolutionary constraint when compared with other genes. Evolutionary constraints are factors which make populations resistant to evolutionary change.
The presence of genes with conserved sexual gene expression dimorphism suggests that they may have important roles during evolution of sex in primates.
Higher evolutionary constraints in the coding regions of female-biased genes as compared to both male-biased and non sex-biased brain expressed genes.
In this context, such evolutionary constraints are present to conserve certain traits in a species. Examples include sexual dimorphism, homology (similarity due to shared ancestry), and body plans.
The investigators explicitly state that for this study they "… do not know what regulatory mechanisms are controling the sexually dimorphic expression of the genes that were identified …" They speculate that expression of some of these genes could be under the regulation of sex hormones. The team took some initial steps to identify the potential conservation of sex hormonal regulation by investigating the presence of estrogen alpha- and androgen response elements. They measured the conserved regions in human and macaque in the genes identified in the study but relegate their results as exploratory, pending further studies in these issues.
The researchers conclude from their observations that some sexual differences in the occipital cortex at the gene expression level may be conserved during the evolution of primates. They cite multiple lines of research that have observed sex differences in behavioral and cognitive abilities in humans and other primates. However, they are cautious and avoid claiming that these differences are caused by biological changes present in the brain — labeling them as "not yet known".
Since further study could yield prevalence data, the study of sex dimorphic genes may in the future shed light on the basis of various birth conditions, including neurological disorders and psychiatric diseases, with differing prevalence between the sexes. Such advvances would be a natural consequence of identifying the precise role of sexually distinct expression profiles and their possible involvement in physiology, behavior and cognition.
[N1] This work was supported by grants from the Börströms Foundation to Jennifer A. Leonard and Elena Jazin, the Swedish Science Foundation to Elena Jazin, and the National Institutes of Health (NIH) to Yoav Gilad.
[N2] The authors of this research use the terms sex and gender interchangeably. All references to gender, whether anatomically external or in the brain, are understood in context as pertaining to sex.
An Evolutionarily Conserved Sexual Signature in the Primate Brain. Björn Reinius, Peter Saetre, Jennifer A. Leonard, Ran Blekhman, Roxana Merino-Martinez, Yoav Gilad, Elena Jazin. (2008). PLoS Genetics 4(6): e1000100. doi: 10.1371 / journal.pgen.1000100 [ Download PDF ]
Abstract
The question of a potential biological sexual signature in the human brain is a heavily disputed subject. In order to provide further insight into this issue, we used an evolutionary approach to identify genes with sex differences in brain expression level among primates. We reasoned that expression patterns important to uphold key male and female characteristics may be conserved during evolution. We selected cortex for our studies because this specific brain region is responsible for many higher behavioral functions. We compared gene expression profiles in the occipital cortex of male and female humans (Homo sapiens, a great ape) and cynomolgus macaques (Macaca fascicularis, an old world monkey), two catarrhine species that show abundant morphological sexual dimorphism, as well as in common marmosets (Callithrix Jacchus, a new world monkey) which are relatively sexually monomorphic. We identified hundreds of genes with sex-biased expression patterns in humans and macaques, while fewer than ten were differentially expressed between the sexes in marmosets. In primates, a general rule is that many of the morphological and behavioral sexual dimorphisms seen in polygamous species, such as macaques, are typically less pronounced in monogamous species such as the marmosets. Our observations suggest that this correlation may also be reflected in the extent of sex-biased gene expression in the brain. We identified 85 genes with common sex-biased expression, in both human and macaque and 2 genes, X inactivation-specific transcript (XIST) and Heat shock factor binding protein 1 (HSBP1), that were consistently sex-biased in the female direction in human, macaque, and marmoset. These observations imply a conserved signature of sexual gene expression dimorphism in cortex of primates. Further, we found that the coding region of female-biased genes is more evolutionarily constrained compared to the coding region of both male-biased and non sex-biased brain expressed genes. We found genes with conserved sexual gene expression dimorphism in the occipital cortex of humans, cynomolgus macaques, and common marmosets. Genes within sexual expression profiles may underlie important functional differences between the sexes, with possible importance during primate evolution.
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Restricting the scientific process to only those theories and conclusions that support our sociological assumptions and rejecting any evidence that does not support our presuppositions leads to inevitable cultural suicide.
Crazy Sexy Cancer. This is the trailer for a documentary film by Kris Carr about her cancer adventure. It premiered on TLC and Discovery Health (Fall 2007).
Kris also authored two books, Crazy Sexy Cancer Tips and Crazy Sexy Cancer Survivor: More Rebellion and Fire for Your Healing Journey.
But could the differences between sexes constitute a signature of sex differences in the brain? "Knowledge about gender differences is important for many reasons. For example, this information may be used in the future to calculate medical dosages, as well as for other treatments of diseases or damage to the brain," says Elena Jazin, Associate Professor, 
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