Autosomes also participate in sex determination. SOX9 gene is an autosomal gene on chromosome It activates the function of TDF factor which is encoded by Y chromosome. TDF factor is critical in male sex determination. Hence, a mutation of SOX9 causes the development of Y chromosome, resulting in a female. Autosomal genetic disorders occur due to either the non-disjunction in parent chromosomes Aneuploidy during gametogenesis or the Mendelian inheritance of deleterious alleles.
Disorders with Mendelian inheritance can either be dominant or recessive Ex: Sickle cell anemia. Figure 1: Human male karyotype. Sex chromosomes are referred to as allosomes. They determine the sex of an individual. The sex determination also happens in most animals and many plants. Humans have only 2 sex chromosomes in their genome which are labeled as X chromosome and Y chromosome. A female individual is determined by XX and a male individual is determined by XY.
A female contains the same two copies of sex determining genes arranged in the same order in both X chromosomes homomorphic. Therefore the sex chromosomes in a female are homologous to each other. During Meiosis , female gametes are made of a single X chromosome plus 22 autosomal chromosomes. Male gametes are made either of an X or Y chromosome plus 22 autosomal chromosomes.
In humans, there is a total of 46 chromosomes or in pair of Out of these, 2 are sex chromosome XX or XY , and 44 are autosomes.
Mice have in all 40 chromosomes, out of which 38 are autosomes, and 2 are sex chromosomes. Abnormalities in the chromosomes autosomes or sex chromosomes results in the retardation in growth of an organism or delay in development. We defined living organisms by the presence of the living cells in them, and it can be multicellular or unicellular. The cell has many organelles which perform their specific function to make the bodywork properly.
Among all the organelles, the nucleus is the most vital or essential part of the cell. The nucleus contains the thread-like structure known as chromosome, which includes the genetic information and is transferred from one organism to other of the same species. Each of the chromosomes is composed of the tightly packed DNA, that is coiled around the histones protein. Although the location and structure of a chromosome may vary among prokaryotes, eukaryotes and viruses, like in prokaryotic cells the chromosome consist of DNA, while non-living viruses the chromosome may consist of either RNA or DNA, in eukaryotes, the chromosomes are enclosed within a membrane-bound nucleus.
With all such information, at this moment we will be highlighting the points that differentiate the autosomes and sex chromosomes, also known as allosomes.
We will also provide a brief description of them. Basis for Comparison Autosomes Chromosomes Meaning Such a pair of chromosomes that regulate the somatic characters of the body are known as autosomes. Finally, the chromosomal positions of rDNA clusters also appeared to be constrained in the multiple-X system.
The autosomal number of clusters was two one pair of allelic copies , but never exceeded this number, and when more than two rDNA copies were found in Cicindela they always appeared on the heterosomes, most commonly on the X only, and in some species a further rDNA copy on the Y chromosome. This is in contrast to the single-X chromosome systems in the basal groups of Cicindelidae which exhibit between four and eight two to four pairs autosomal rDNA clusters [ 25 ].
The nature of these constraints on the karyotype remains unknown but they may be linked to the evolutionary stability of this multiple-X system [ 25 ].
Despite the morphological conservation of the multiple-X system, the frequent movements of genes between autosomal to heterosomal positions expose the affected loci to greatly altered dynamics of gene evolution and recombination. As there is no cross-over in the male heterosomes in cicindelids, rates of homologous recombination in the sex chromosomes are reduced by half for the X chromosomes recombination only in females and to virtually zero for the Y resulting in their inevitable degradation; [ 13 ].
The hemizygous nature of the X will greatly increase selection on recessive mutations, altering the rate and kind of mutational changes. This would cause the rearranged genes to diverge quickly, even if rearranged gene regions are duplicated.
In the case of the rDNA clusters, this could reduce the rate of homogenization of copies in different parts of the genome. This supports the idea that the translocation to the sex chromosomes results in changes of evolutionary dynamics. These kinds of chromosomal rearrangements might also have an effect on speciation. Translocations of genes between autosomes and sex chromosomes will greatly change the possibilities for gene flow, and changes in the number of X chromosomes may alter the production of balanced gametes.
Incompatibility of gametes with different numbers of X chromosomes, or indeed changed position of rDNA clusters, could lead to incorrect separation of chromosomes during anaphase I in a hybrid, and produce a number of unbalanced gametes resulting in reproductive disadvantage. In the case of changes in X chromosome numbers this effect could be exacerbated by altering the sex determination control and the gene regulation associated with changes in heterosome number.
As pointed out in the recent literature [ 1 , 6 , 7 , 47 ], it is not likely that these 'underdominant' variants become established in a population. This has provided a strong argument against the stasipatric model of speciation [ 5 ] which suggests that chromosomal rearrangements cause reproductive isolation due to hybrid dysfunction.
However, when involving sex chromosomal unidirectional rearrangements as those in Cicindela , this model may still be valid. Depending on the precise genotypes participating in a mating, the combination of certain gametes could lead to a significant proportion of inviable e.
These effects may be exacerbated by the reduction of gene flow from suppressed recombination and extending the effect of linked isolation genes, a mechanism proposed as the main driver for the fixation of novel karyotypes under more recent models [ 8 , 48 ].
As gene flow is more restricted between sex chromosomes than autosomes, sex linked genes are particularly efficient to produce such postzygotic barriers [ 49 ], and hence rearrangements involving sex chromosomal portions of the genome may be a particularly effective isolating mechanism.
Therefore, the high level of apparent chromosome rearrangements and the deposition of a substantial portion of the genome in the low-recombining sex may promote speciation in Cicindela. With some 1, species world-wide, this is one of the largest genera of insects.
In particular, the observation of evolutionarily short-lived X 1 X 2 Y and X 1 X 2 X 3 X 4 Y lineages suggests that these chromosomal changes could initiate reproductive isolation. If these rearrangements are frequent relative to other ecological or geographical processes influencing speciation rates, the cytogenetic parameters could drive speciation and possibly be responsible for the great species richness in Cicindela.
In addition, the population structure of Cicindela is also favoring the fixation of chromosomal mutations locally, as most species are early succession specialists frequently occurring in isolated habitat patches where a dynamic system of colonization and extinction may enhance the separation of local genetic entities.
In support of this possibility, we found two Iberian species, C. The evolutionary significance of elaborate multiple-X systems compared with simpler sex chromosomes is still poorly understood. Phylogenetic approaches can greatly increase the power of comparative cytogenetic analyses, and have revealed the great fluidity of the cicindelid multiple-X chromosomes, while also establishing evolutionarily conserved features.
Even more variable than the X-chromosome numbers are translocations of the rDNA clusters. This provides the background for future investigations to understand the evolutionary forces operating on the sex chromosomes. Whereas the current study uses a macroevolutionary approach, establishing the framework of character variation over greater evolutionary distances, this can be combined with the specific effects of rearrangements on the population level.
Sex chromosomes are of specific interest to speciation, not least because they have been shown to accumulate genes determining species specific traits such as host plant use and pheromone response in butterflies [ 50 ]. The observed cytogenetic phenomena should also be studied because of their functional consequences, with regard to the control of sex determination, chromosome size and morphology, and the mechanisms of gene repositioning.
Further investigation will require a targeted approach to the comparative genomics of tiger beetles and the construction of chromosomal homology maps, using reciprocal chromosome painting with sex chromosome specific probes obtained by microdissection.
Molecular cytogenetics studies, assisted by comparative genomics and phylogenetics, may help to investigate the evolutionary dynamics of gene content of chromosomes and may reveal karyotypic changes that remain unnoticed in conventional cytogenetics analysis.
These studies are the basis for tests of how variation in sex chromosomes can drive population differentiation and speciation rates. Taxon sampling for this study was limited to Cicindela from North America.
There are some described species recorded for the North American continent, grouped in 11 subgenera [ 51 ]. A representative sample of all major clades in the mtDNA tree was selected for chromosome analysis, with good representation of all four endemic radiations. Character variation in X chromosome number and rDNA localization was assessed on this tree, using parsimony optimization as implemented in MacClade [ 52 ].
Adult beetles were obtained in the field, and chromosome preparations were obtained from male gonads. Mitotic and meiotic chromosomes can be obtained at different stages of development in the tubular testes, and can be observed as described previously [ 28 ]. Active NORs i. Proc Natl Acad Sci. Annu Rev Ecol Syst. Article Google Scholar. Freeman, Google Scholar. Butlin RK: Recombination and speciation. Mol Ecol. Spirito F: The role of chromosomal change in speciation. Endless forms: species and speciation.
Navarro A, Barton NH: Chromosomal speciation and molecular divergence - Accelerated evolution in rearranged chromosomes. Am J Hum Genet.
Rice WR: Sex chromosomes and the evolution of sexual dimorphism. White MJD: Animal cytology and evolution, 3rd ed. Bachtrog D: A dynamic view of sex chromosome evolution.
Article PubMed Google Scholar. Chromos Res. G-, C- and chromosome replication banding. Can J Zool. Gas Exchange 5. Homeostasis Higher Level 7: Nucleic Acids 1. DNA Structure 2. Transcription 3. Translation 8: Metabolism 1. Metabolism 2. Cell Respiration 3. Photosynthesis 9: Plant Biology 1. Xylem Transport 2. Phloem Transport 3. Plant Growth 4. Plant Reproduction Genetics 1. Meiosis 2.
0コメント