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Classification and Systematics
Published in Jacques Derek Charlwood, The Ecology of Malaria Vectors, 2019
In addition to species-specific fixed inversions, a high degree of chromosomal polymorphism, in the form of paracentric inversions (i.e., excluding the centromere-inversions that include the centromere are called pericentric inversions), has been described in natural populations of A. gambiae. They result from a double break in a segment of the chromosome with end-for-end rotation of the fracture between the fracture lines and re-fusion of the fragments. Inversions may not cause abnormalities in the carriers (as long as there is no genetic loss or extra genetic information), but heterozygotes (in the case of A. gambiae species these may be hybrids) may be at a disadvantage. The ecotypic speciation model is founded on the observation that certain paracentric inversions that are polymorphic in A. gambiae are nonrandomly distributed in nature. They occur most often on the right arm of chromosome 2 (2R). These are thought to contain multilocus genotypes that are adaptive to specific ecological niches. Under this model, populations carrying alternate gene arrangements would inhabit different, spatially isolated, habitats. Genetic divergence, enhanced by reduced recombination associated with the inversions, would then evolve. Ultimately, divergence would include genes resulting in different mating behaviour, in particular different swarming behaviour, and so reproductive isolation would ensue. Hybrid disadvantage may be overcome if one species transfers genes that may be under considerable selective pressure – for example, genes that confer resistance to insecticides. Should hybrids mate with the susceptible species then the resistance genes will spread into the second species, as appears to be the case in Ghana (Clarkson et al., 2014). Other genes that might ‘hitchhike’ with the adaptive genes tend to be actively selected against and are eventually removed from the population (Hanemaaijer et al., 2018).
Heterochromatin extension: a possible cytogenetic fate of primary amenorrhea along with normal karyotype
Published in Journal of Obstetrics and Gynaecology, 2022
Bishal Kumar Dey, Shanoli Ghosh, Ajanta Halder, Somajita Chakraborty, Sanchita Roy
The region of heterochromatin also acts as a key part in chromosome structure, histone modification and gene regulation. There is evidence from where we come to know that there may be displacement of heterochromatin from one chromosome to another. Perhaps, this displacement is helping in the extension of a particular chromosome at the heterochromatin portion of the long arm (Bannister and Kouzarides 2011). The mechanisms of spindle fibres, chromosome movement, meiosis crossover and change of sister chromatids are considered to be the integral region as heterochromatin for a chromosome. At the time of meiosis, there may be a change in area of synapses of homologous chromosomes in the polymorphic heterochromatin region. The heterochromatin in chromosomal polymorphism can also regulate gene expression by reversible transformation between heterochromatin (non-coding DNA sequences) and euchromatin (expressed DNA sequences) thus justifying certain clinical expression like short stature or PA. It was also postulated that defective histone protein methylation due to presence of heteromorphic variants may play a more crucial role in ovarian failure. Association of heterochromatin polymorphism with ovarian dysgenesis may be a reason for the occurrence of PA. For that, we need to study on a greater number of patients on the basis of their nucleosome’s functionality and heteromorphic polymorphism by sequencing.