Sex Linkage and Sexual Dimorphism
Sexual dimorphism is the phenotypic differences between sexes within the same species. In 1980, Lande suggested that sexual dimorphism occurred in two phases. In phase I, the male and female phenotypes will continue to diverge until they become equal and opposite. Phase II entails the development of the optimal male trait and the return of the optimal female trait, prior to selection. It is thought that to determine how sexual dimorphism has evolved within a species, one must evaluate the intersex genetic correlation and sexual selection of both male and female traits (4). Sexual dimorphisms can directly result from differences in the autosomal or sex chromosomes. The measure of variance, either from sire or dam, can be estimated depending on whether it is inherited on a sex chromosome or an autosome. A difference in variation among the sexes is only seen when a gene is on the sex chromosomes. However, it is important to remember that phenotype often depends on both genetics and environment. Many traits are expressions of multifactorial events that involve multiple genes and environmental factors.
Gender is determined as soon as fertilization occurs. Therefore, genetic differences between the genders must be present as a result of carrying different genes and having different dosages of gene product. The X chromosome is most often found to be larger in size and carrying more genes; however this is not always the case among all species. One study is using the species Silene latifolia to investigate the divergence of sex chromosomes of a species at an early stage in evolutionary development in an effort to possibly relate their findings to what occurred in humans many years ago. It has been found that the Y chromosome in this species still contains many euchromatic regions, one interestingly being the pseudoautosomal region that is seen on the human Y chromosome. The Y chromosome of S. latifolia has not begun to degenerate yet. This paper looks specifically at a gene known as DD44. It is found on both sex chromosomes, but on different arms, which was determined to result from chromosomal rearrangement. It is suggested that chromosomal rearrangement played a factor in the evolution of human chromosomes as well (10). It is also known that the sex chromosomes, X and Y, do have comparable regions, like the pseudoautosomal region, but the Y chromosome, the sex-determining chromosome, does carry genes that are not located on the X, such as the sex determining region Y, SRY located on the Y chromosome. Therefore, for some traits, like gonadal development, it has been found that the male’s phenotype is due to the expression of a particular gene, whereas the female’s phenotype is the result of lack of that expression. Initially, it was found that the testis-determining factor, TDF, is located on the short arm of the Y chromosome and its expression is responsible for inducing the male traits. If an XY individual lacks the short arm of the Y chromosome, this individual would appear to be female. However another gene located on the short arm of the Y is the SRY. Evidence has suggested that the SRY may actually be the TDF (7). Research has shown that loss of this gene also results in a female phenotype and if the SRY translocates to the X chromosome, a hermaphroditic individual is created (7).
The second factor concerning sex chromosomes is dosage or the number of alleles for each loci producing gene product. Female cells, having two X chromosomes, go through a process known as X inactivation early in fetal development as a means of dosage compensation, whereas males do not since they are hemizygous. Even though both males and females produce the same amount of gene product, females tend to phenotypically adjust to mutations within genes on the X chromosome less drastically than males because of random X inactivation. This phenomenon allows some cells to produce the mutant allele and others to produce the normal allele within the female. However, since males only have one X chromosome, the mutated allele will always be observed within the affected individual. This is another important factor concerning variation in expressivity, especially when it comes to differences in the phenotypes of individuals with X-linked disorders.
One study in particular, where dosage appears to be important, looks at the differences in expression between the SRY genes on the Y chromosome and their homologous pairs on the X chromosome. They found variation in expression levels of five of the genes, which most likely are responsible for the differences in neural development patterns observed in mouse brain tissues between the genders at different life stages. It seems that the Y-linked homologue could not compensate for the female’s extra X-linked allele (19).
Differences in expression of genes located on autosomes also leads to sexual dimorphism. It has been suggested that expression of protease nexin-1 and vanin-1 may play a role in the somatic development of the testis. The genes that encode these proteins are found to have male-specific expression before gonadal differentiation in the fetus (5). Another autosomal gene produces MIS, which is a protein that may have a role in genitalia development by affecting androgen receptors. The genes responsible for the production of steroids, such as testosterone, are also located on autosomes. It is known that steroids play a key role in sexual development in both sexes (7).
Abdominal pigmentation is a prime example of expressivity resulting from the interaction of multiple genetic factors. It has been observed that abdominal pigmentation in Drosophila spp. has been correlated to expression of genes on the X chromosome, as well as on the three autosomes. In the Drosophila dunni subgroup it has been found that there are three pigmentation pathways consisting of activator and repressor genes. The pathway that correlates to the mid-dorsal region is found to be X-linked. However it is important to note that autosomal factors play a role in the phenotypic outcome of this region. The two remaining pathways that determine abdominal pigmentation in Drosophila dunni are controlled by autosomal genes. Interestingly enough this paper also addresses how the degree of expression in the offspring is correlated to the amount expressed by their parent of the same sex. The posterior end seemed to have a sex-limiting expression pattern. The male’s phenotype greatly depended on that of his father’s phenotype (8). Another study compares the differences in pigmentation between the sexes and among two species within the Drosophila genus. Their results suggested that the X chromosome was responsible for about 90% of the varying pigmentation patterns observed. Comparing the differences among species, they were able to conclude that there must be some autosomal factors affecting pigmentation because in some species both genders lacked pigmentation, leading them to believe that their specific species did not express a specific autosomal gene. Natural selection could have been acting on these species causing the phenotypic differences among them (9).
Hormones also have a great effect on reproductive behavior and function. Neural receptors bound by targeted hormones induce function through a signaling cascade. Mutations within genes encoding for receptors affecting hormone levels have major effects on reproductive development. For example, SRC-1 is a protein that is important in male development because it interferes with estrogen, a feminizing hormone (2). SRC-1 defeminizes brain development correlating with sexual differentiation of sexual behaviors. However, androgen action is necessary for proper sex differentiation and reproductivity in males. A decrease in males, as well as an increase in females, can lead to sexual disorders. Another paper discusses how defects in the androgen receptor can occur since both testosterone and dihydrotestosterone both bind the same receptor protein. Development of specific sexual characteristics depends upon which complex is formed. The androgen receptor in humans is X-linked (7). Also defects such as steroid 5alpha-reductase can cause clinical manifestations. This steroid is an important factor in forming dihydrotestosterone. Affected individuals will have an intersex phenotype due to deficiency in this particular steroid (7).
Genotypes are only partially responsible for displayed phenotypes within individuals. Environment is the other critical factor that has a major effect on phenotype. As DNA replicates, mutations are randomly incorporated into the genome and creating variation among individuals within the same species. Natural selection can only occur if there is variation among the individuals. However, once this occurs, environmental stresses begin to select against less fit traits by causing affected individuals either to have difficulty reproducing and or causing them to die. An example of selective pressures affecting sexual dimorphism is observed in the household finch. This study observes the finch as a means of further understanding the evolution of sexual dimorphism. It suggested that the main factor was how each sex responded to selective pressures within their environment. It was also concluded that neither gender had reached their optimal fitness because of their environment, leaving room for variation within inheritable, sexually morphological traits. In another study, the lizard’s trunk size and head size are compared between the sexes. It is thought that these traits, along with many others, are affected by a combination of sexual selection and fecundity selection. They observed a major difference in trunk length of females. A larger trunk length is advantageous to the females by allowing her to carry more eggs. In males, larger head size seemed to be advantageous by allowing them to protect themselves against other males, which increases their chances of propagating (11).
Sometimes, phenotype not only depends on the gene inherited, but also depends on which parent the gene was inherited from. Imprinting, which is due to permanent chemical modifications of base pairs, can also have a major effect on genetic inheritance by silencing one of the alleles. Genes that are imprinted are often involved in fetal development or when dosage is an important factor. It is observed that males are more likely to develop autism than females. A study discusses the effects of imprinting on the X chromosome and its possible involvement in autism. It is hypothesized that there is a phenotypic threshold that must be met in order to present clinical manifestations and this threshold is controlled by a gene located on the X chromosome. This gene is imprinted in the maternal X, but not on the paternal X chromosome. However since females are the only ones that receive a paternal X, they have a higher threshold (15). It was found in stalk-eyed flies that the male eyespan and female body length depended on the genotype of the maternal X (18). In the insect world, individuals are often segregated by a caste system of queens and workers. In a specific ant species studied by Volny et al., the workers were sterile, whereas queens had reproductive capabilities. Prior studies reported that this occurrence is attributed to environmental stress; however this study felt that the answer lied within a specific locus in their genome. Queens seem to derive from same lineages, and are homozygous for caste, whereas workers are heterozygous. This paper reflects on how this type of population growth can affect how effectively a colony adjusts to its environment (16). Another study done by Savalli et al. discusses how male body size of the beetle affects its ejaculate mass. The size of this mass is an inheritable trait and is positively correlated with the size of the eggs laid by the female (13).
Plant species with separate sexes have evolved from hermaphroditic ancestors. Theoretically, it is thought that females have evolved first and as the frequency of females increases, the female traits within the hermaphroditic individuals will be selected against and the male traits will become more and more predominant. This would finally result in the formation of two separate genders. For this to occur there must be genetic variability within the species and there must be natural selection against hermaphroditic individuals within their environment. Using a wild strawberry species, one study supports the theory that there is a phenotypic trade-off between male and female function in hermaphrodites concerning traits such as the amount of pollen per flower and fruit set. Males were found to have low levels of genetic variation of pollen per flower. Also it was suggested that there are still traits that are not distributed between the sexes, but rather are present in both (1). Another group conducted its study observing the evolution of shrimp. Weeks et al. studied an unusual inbreeding species where males are mating with hermaphrodites. This had a wide range of effects on the resulting offspring that was dependent on the diversity of the population. The more diverse population had lower fitness, whereas the less diverse population had higher fitness (17).
Sexual dimorphism mainly stems from genetic variations located on both sex chromosomes driven by sexual selection. However, it is important to remember that autosomal genes, transcription modification, such as imprinting, and environmental stress also contribute greatly to the variance of some characteristics. Hormones, which are gene products, are also key factors in sexual development and mutations within them can have drastic effects. Evolution of sexual dimorphisms is important for survival of each species allowing each gender to be well fit for their environment even if it means that they do not reach their optimal fitness.
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