Strain-specific variation in the pattern of caudal papillae in Caenorhabditis briggsae (Nematoda: Rhabditidae); implications for species identification

Abstract

Caenorhabditis briggsae is a member of the ‘elegans group’, a monophyletic clade of seven species within Caenorhabditis (SudhausK Nigon & Dougherty, 1949; Sudhaus & Kiontke, 1996). C. briggsae and C. clavopapillata can be distinguished from other elegans group members based on their patterns of caudal papillae (Kreis & Faust, 1933; Nigon & Dougherty, 1949; Friedman et al., 1977). Despite the general utility of caudal papillae patterns as diagnostic characters, their use to discriminate between C. briggsae and C. elegans came into question when a strain of C. briggsae, PB800, was obtained that displayed an elegans pattern at a high frequency. The canonical C. briggsae arrangement of caudal papillae is a 2/4+3 pattern with the third and fourth pair frequently being fused together (Nigon & Dougherty, 1949; Fig. 1A). This pattern also was reported for a second C. briggsae strain, AF16 (Fodor et al., 1983). The C. briggsae arrangement differs from those of C. elegans and C. remanei which have a 2+(1)+3+3 pattern (Maupas, 1900; Sudhaus, 1974; Baird et al., 1994; Fig. 1B, C). Immediately after its establishment, 70% of PB800 males displayed an elegans pattern on both sides. With subsequent inbreeding, this frequency decreased but still remained relatively high (see below). To address their use as diagnostic characters, caudal papillae patterns of AF16, PB800 and three additional C. briggsae strains, HK104, HK105, and VT847, were determined by microscopic observations using differential interference contrast optics (magniŽ cation 400£). These strains were established from collections at disparate lo-