Abstract
The horn fly, Haematobia irritans is an obligate haematophagous cosmopolitan insect pest. The first reports of attacks on livestock by Haematobia irritans in Argentina and Uruguay occurred in 1991, and since 1993 it is considered an economically important pest. Knowledge on the genetic characteristics of the horn fly increases our understanding of the phenotypes resistant to insecticides that repeatedly develop in these insects. The karyotype of Haematobia irritans, as previously described using flies from an inbred colony, shows a chromosome complement of 2n=10 without heterochromosomes (sex chromosomes). In this study, we analyze for the first time the chromosome structure and variation of four wild populations of Haematobia irritans recently established in the Southern Cone of South America, collected in Argentina and Uruguay. In these wild type populations, we confirmed and characterized the previously published “standard” karyotype of 2n=10 without sex chromosomes; however, surprisingly a supernumerary element, called B-chromosome, was found in about half of mitotic preparations. The existence of statistically significant karyotypic diversity was demonstrated through the application of orcein staining, C-banding and H-banding. This study represents the first discovery and characterization of horn fly karyotypes with 2n=11 (2n=10+B). All spermatocytes analyzed showed 5 chromosome bivalents, and therefore, 2n=10 without an extra chromosome. Study of mitotic divisions showed that some chromosomal rearrangements affecting karyotype structure are maintained as polymorphisms, and multiple correspondence analyses demonstrated that genetic variation was not associated with geographic distribution. Because it was never observed during male meiosis, we hypothesize that B-chromosome is preferentially transmitted by females and that it might be related to sex determination.
Highlights
The economic importance of the horn fly, Haematobia irritans (Linnaeus, 1758) (Diptera: Muscidae) is based on its role as an obligate bloodsucking ectoparasite that plagues cattle around the world (Palmer and Bay 1981, Williams et al 1985, Valério and Guimarães 1983)
Insecticide resistance is one of the best examples of rapid microevolution found in nature (Silva et al 2012) and has strong economical implications
Populations of H. irritans develop in these countries from early spring (October) to the beginning of fall (Guglielmone et al 2001, 2002, Tarelli 2004), with the average degree of infestation depending on the climate
Summary
The economic importance of the horn fly, Haematobia irritans (Linnaeus, 1758) (Diptera: Muscidae) is based on its role as an obligate bloodsucking ectoparasite that plagues cattle around the world (Palmer and Bay 1981, Williams et al 1985, Valério and Guimarães 1983). Many local populations from Central and North Argentina demonstrated resistance to fenvalerate and pyrethroids (Torres et al 1996, Sheppard and Torres 1998, Guglielmone et al 2001), indicating significant changes in the descendants of the original invading stock that could make them even more invasive and destructive in the future. Given the widespread presence of this pest in South Brazil and Paraguay (Sheppard and Torres 1998), and the prevalent cattle trade among these neighbouring countries, the horn fly population in Argentina is likely to be subjected to gene flow
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