Laboratory colonization of one mosquito species and cytogenetic analysis of two genera and four species in the Myrick Marsh floodplain of La Crosse, Wisconsin

Abstract

The objectives of this study were: to gain basic information about mosquito chromosome number and morphology; to find out how and to what extent the chromosomes of genera and species found in a geographical area differ from each other and from those areas, genera and species previously examined; and to establish a laboratory colony of a field collected species. Myrick Marsh was chosen as the sampling area because of its annual inundation with the La Crosse River flood waters and its corresponding diversity of vegetation. Such conditions were deemed favorable for mosquito production. Twenty sites were selected based on accessibility and unique habitats. Collection was by the standard dipper method. Larvae were used for karyotyping while pupae were used for colonization. Initial colonization of aedine species was unsuccessful while a field collected Culex pipiens was established and is presently maintained in the laboratory. In preliminary karyotyping attempts the squash technique first developed by plant cytologists and adapted for mosquito studies by Osmond P. Breland in 1959 was used. This method proved ineffective. A second method using colchicine pretreatment was tried. The results improved but the karyotypes were not usable for proper analysis. This latter method was then modified, tried, and was successful. Analysis of the chromosomes of Aedes cinereus, A. excrucians, A. vexans, and Culex pipiens showed uniformity in chromosome number, the diploid chromosome number being six. The chromosomes were numbered I to III according to total length; I being the smallest, III the largest. With the exception of chromosome I in Culex pipiens, all chromosomes demonstrated metacentric centromere positioning. Heteromorphic sex chromosomes were not found in the four species examined. Although the chromosome number was uniform, species and genera identification were readily accomplished by using a ratio method of analysis (I/II + III). The ratios found in this study were comparable to those of other investigators for some of the same species.