Chromosomal Aberrations in Polytene Chromosome of Anopheles Stephensi

Abstract

The present studies deals with effect of environment on the vectoral status and genotype of Anopheles stephensi (Diptera: Culicidae). Comparison of chromosomal polymorphism in the urban and rural population of the species. A large number of different populations of A. stephensi may have to be examined cytologically for knowing the differences in chromosomal variability which might exist between vector and non-vector populations & those that are susceptible and resistant to insecticides, because vectorial capacity and insecticide resistance are pre-adaptive genetic phenomena with a base in the chromosomes. A well spread salivary chromosomes complement of A. stephensi is comprised of three synansped polytenes elements, attaché at the region of their centromeres, forming a heterochromatic chromocenter. They include a short telocenric/ areocentric X – chromosome is the shortest and appears isolated in the squash preparations, whereas 2R and 2L show attenuation between them. The average measurements of the chromosomes are X- chromosome - 65µ, 2R-270 µ and 2L-170 µ, 3R-220µ and 3L-230µ. While mapping the  chromosomes, entire complement has been divided into 46 zones in which each zone can be identified by characteristics bands or puffs. In the present study a total of 20 cases of various aberrations were observed out of which 18 were inversions; 8 ectopic pairing, 1 asynapses and 2 cases of breakage. Inversions are the most common of all the aberrations. The X-chromosome had only one inversion chromosomal arms 2R had 3 and 2L had 8, 3R had 3 and 3L had 4 inversions. All the inversions accept one were restricted to autosomes, while the X-chromosome seems resistant to structural changes. This shows the genetic balance that has been achieved by the (sex. chromosome) is involved, the effect is more deleterious as it causes sex ratio distortions and specificity in the natural populations. In the present investigations all the inversions fail in the category of "floating inversions” none was encountered more than once in about 100 preparations studied for the purpose. Some intrachromosomal and interchromosomal ectopic pairings were observed between different chromosomes. The phenomenon of synapses was observed in 2L, where effected subzones were 15A-15C.