Spontaneous Triploidy in the Black FlySimulium SnowiStone & Snoddy (Diptera: Simuliidae)

Abstract

Polytene chromosomes in the silk glands of larval black flies are large and distinctly banded (Rothfels and Dunbar 1953), facilitating detection of polyploid individuals. Spontaneous triploids are observed in diploid black fly species ap prox imately once per >1500 chromosome preparations (Adler et al., 2004). Lateinstar larvae are generally used to make these preparations, and the actual rate of spontaneous triploidy possibly is higher because triploids that terminate earlier in development are not seen. Here, I report a triploid larva of Simulium snowi Stone & Snoddy. Larval black flies were sampled from Bluewater Creek, Lauderdale County, AL (34°54.077' N, 087°26.383' W), and fixed in Carnoy’s solution (3:1 etha nol:acetic acid) on 13 November 2010. Slide preparations were made of poly tene chromosomes from 10 larvae, following the Feulgen staining method (Rothfels & Dunbar 1953, Adler et al., 2004). One larva of S. snowi with male gonads was triploid (Fig. 1). This individual could possibly be a hybrid; I have found no fixed chromosomal banding differences between S. snowi and the sympatric S. fibrinflatum Twinn, and larvae of the two species are isomorphic (Adler et al., 2004). However, the close association of all three sets of chromosomes suggests that this larva is a spontaneous autotriploid, representing the first record of a triploid in the S. jenningsi species group. Spontaneous triploids have been reported from at least 6 species in the genus Simulium throughout the world, including Australia, Russia, and the United States (Pasternak 1964, Chubareva et al., 1974, Bedo 1977, Adler 1986, Adler et al., 2004). Spontaneous triploids are typically reproductive dead ends (Adler et al., 2010), but 8 triploid black fly species are known worldwide, with four par theno genetic triploid species in North America (Adler et al., 2004). Of these, Gym no pais dichopticoides Wood, Gymnopais holopticoides Wood, and Pro si mu li um ursinum (Edwards) are allotriploids (Adler et al., 2004), but Stegopterna mu tata (Malloch) is possibly an autotriploid (Currie and Hunter 2003). Spontaneous triploid specimens have contributed to an understanding of the biology of black flies. For example, it was discovered that the sex of a black fly depends on the presence or absence of a male-determining factor because XXY triploids are male, whereas XXX triploids are female (Rothfels 1980, Adler et al., 486 ENTOMOLOGICAL NEWS