How Platypus Sex Chromosomes and Bird Sex Chromosomes Linkage may have Same Origin
The determination of sex for several species has been genetic with females and males bearing different genes/alleles specifying their sexual morphology. Sex determination in animals in most cases goes hand in hand with differences in chromosomes through haplodiploidy or chromosomal combination of ZW, ZO, XY, and XO. This difference is often accelerated by a sex locus (main gene) accompanied by several other genes in a domino effect (Smith & Voss, 2007). There has been a generally accepted argument that the ZW and XY sex chromosomes underwent evolution in two separate branches at the same time (Namekawa & Lee, 2009).
However, there is substantial proof suggesting that there is the possibility of initial transition between the XY and ZW sex chromosomes like in the case of Xiphophorus maculatus bearing both XY and ZW systems within the same population despite the difference in location of genes between the ZW and XY sex chromosomes (Veyrunes et al, 2007). Due to the DMRT 1 gene found in the platypus X chromosomes that is usually possessed by birds, there is substantial reason to support the possibility of a common link between ZW and XY sex chromosomes (Smith & Voss, 2007) as cited in (Jennifer, 2006).
Smith and Voss argue that the proportion of chicken Z and human X orthologs noted in ALG2 is far way beyond what would be expected to be by chance, disputing the hypothesis currently in acceptance arguing that there are separate autosomal origins. Results obtained from research suggested that mammals and bird’s sex chromosomal regions originated from a common ancestral chromosome (Deakin, Hore, Koina, & Graves, 2008; Smith & Voss, 2007).The mammalian males have a combination of X and Ychromosomes to form the XY chromosomes while the females have two similar XX sex chromosomes. Comparing to birds, which have the ZZ-ZW sex determination system, their females are classified with heterogametic ZW sex chromosomes (Smith & Voss, 2007; Namekawa & Lee, 2009).
There is also notable similarity between the chicken W chromosomes and the mammalian Y chromosomes in their conspicuously smaller size as compared to their Z and X counterparts respectively, and also that they both contain fewer loci (Deakin et al., 2008; Smith & Voss, 2007). Deakin et al. (2008) further argues that as much as the mammalian X and Y chromosomes share homology between coding genes on the Y and X, and also within pseudoautosomal regions, the Y chromosomes are significantly variable and smaller in size as compared to the X. It is thus argued that this proves the theory that supports the evolution of heteromorphic sex chromosomes from a homologous autosome pair in a mammalian ancestor after the acquisition of a sex determinant locus by one member of the pair, which resulted in suppression, differentiation and recombination between members of the pair (Deakin et al., 2008).
The evolution of the Z and W bird chromosomes are also proposed to have evolved in a similar way from an ancient autosomal pair. This is supported by the high similarity between the ZW and XY chromosome properties. For instance, both the mammalian X chromosome and bird Z chromosome are highly conserved between species with the W chromosome being degraded in different groups of birds to varying extents (Deakin et al., 2008). Both the mammalian and bird X and Z chromosomes respectively are large in size, conserved between species, and contain many genes. They also appear to be sex-biased in their gene content (Deakin et al., 2008).
Veyrunes et al., (2008) test the hypothesis that argues the possibility of the bird ZW and mammalian XY system sharing homology with the platypus sex chromosomes. The research uncovers that homologous regions to the chicken Z chromosomes were evident, distributed sparsely throughout, principally on the X3 and X5 (Veyrunes et al., 2008).
The research conducted in this coursework was based on secondary sources of data that featured how platypus sex chromosomes and bird sex chromosomes linkage could have shared the same origin. The research uses three major sources to evaluate the possibility of the existing platypus and bird sex chromosomes sharing the same origin. These sources are selected on the basis of originality, extent of research conducted, methods of data analysis, methodology used in the research and authority of the authors, population size used in data analysis, and relevance to the study. This coursework reviews these sources, analyzing the arguments of the authors in relevance to the topic of discussion outlining their results and findings and providing a brief discussions to support the results.
Results and findings
Veyrunes et al. (2008) localizes and identifies BACs on the sex chromosomes on platypus. Male platypus mitotic metaphase chromosomes resulted from the hybridization of 32 BAC clones. Out of the 32, 18 mapped on a single X chromosome: two were mapped on X5p, two on X3q, five on X5q, and nine on X1q, while the remaining 14 had signals with both the Y and X chromosomes (Veyrunes et al., 2008). The extent of homology between human chromosome X and platypus chromosome 6 was confirmed by the results obtained by Veyrunes et al., previously established for seven XCR genes. Results obtained depicted the entire representation of the conserved region of the therian chromosome X, by platypus chromosome 6, which has been reorganized in line with chicken chromosome 4p, human X chromosomes, and opossum X (Veyrunes et al., 2008).
Results obtained from another study, which tested for the origin of mammal and bird sex chromosomes through mapping amniote sex chromosome loci locations in a salamander were as follows. Orthologs of 20 amniote sex chromosomes were meiotically mapped to ambystoma linkage groups. Most of the Z orthologs were found to map on a single ambystoma linkage group (ALG2). The results had a frequency of Gadj = 6.2, P = 0.013 which is far way beyond than would be expected by chance. Also, the XCR orthologs frequency on ALG2 was above what would be expected by coincidence, Gadj = 6.3, P = 0.009 (Smith & Voss, 2007). When ALG2 genes were searched against the full genome assemblies for chicken and human, they were found to be randomly distributed among chicken chromosomes with Gadj = 32.9, P = 9.7e-9 and human chromosomes with Gadj = 42.6, P = 6.9e-11, which hindered reciprocal amniote sex-chromosome loci (Smith & Voss, 2008).
Distribution of orthologies of sex chromosomes on ALG2 has been demonstrated to be beyond the likelihood of having occurred by chance through statistical analysis. The idea of the Z-W and X-Y chromosomal regions being linked to a prior ancestral chromosome of the amniote and amphibian lineages is inconsistence with the pattern of orthologies on ALG2 (Smith & Voss, 2008). Analysis done in the study showed that quite a good number of gene orders that were conserved between the autosomes and sex chromosomes of humans and chickens were conserved on the same chromosome of a salamander and interspersed. This implied that some of the ancestral chromosome gene content that gave rise to the Z and X sex chromosomes are still retained by the ALG2 (Smith & Voss, 2008).
Various reasons have been outlined in this review in support of the link between platypus and bird sex chromosomes arguing that there is a higher probability that they share the same origin. Among the reasons outlined include their similarities, which is beyond what would be expected to occur by chance. Taking for instance, there are significant similarities between the chicken W chromosomes and the mammalian Y chromosomes in their conspicuously smaller size as compared to their Z and X counterparts respectively, and both contain fewer loci.
The evolution of the Z and W bird chromosomes are also proposed to have evolved in a similar way from an ancient autosomal pair, which is supported by the high similarity between the ZW and XY chromosome properties. For instance, both the mammalian X chromosome and bird Z chromosome are highly conserved between species. In addition, both the mammalian and bird X and Z chromosomes respectively are large in size, conserved between species, and contain many genes. They also appear to be sex-biased in their gene content.
There was high similarity found between the variable dosage and partial relationship of the Z chromosome genes on chicken and that of multiple platypus X chromosomes. There was also shared similarity in dosage compensation features between the mammal X chromosomes and bird Z chromosomes with the platypus. Basing on the above provided evidence, one could possibly and confidently defend that there is substantial evidence to demonstrate that platypus sex chromosomes and bird sex chromosomes may have shared the same origin.
Deakin, J. E., Hore, T. A., Koina, E., & Graves, J. A. M. (2008). The status of dosage compensation in the multiple X chromosomes of the platypus. PLoS genetics, 4(7), e1000140.
Namekawa, S. H., & Lee, J. T. (2009). XY and ZW: is meiotic sex chromosome inactivation the rule in evolution? PLoS genetics, 5(5), e1000493.
Smith, J. J., & Voss, S. R. (2007). Bird and mammal sex-chromosome orthologs map to the same autosomal region in a salamander (Ambystoma).Genetics, 177(1), 607-613.
Veyrunes, F., Waters, P. D., Miethke, P., Rens, W., McMillan, D., Alsop, A. E., & Graves, J. A. M. (2008). Bird-like sex chromosomes of platypus imply recent origin of mammal sex chromosomes. Genome research, 18(6), 965-973.