Ray pattern is one of the most variable morphological characters in rhabditid nematodes. Within Caenorhabditis , a modified ray pattern in which ray 3 is posterior of the anus and frequently fused with ray 4 is diagnostic for C. briggsae . This diagnostic character is based on observations of a limited number of C. briggsae strains. Recently, a C. briggsae strain, PB800, was obtained that exhibited an elegans ray pattern at a high frequency. Prompted by this observation, ray pattern variation has been assessed in four other C. briggsae strains, AF16, HK104, HK105, and VT847. AF16 and VT847 exhibited predominately the canonical briggsae pattern whereas HK104, HK105, and PB800 exhibited briggsae and elegans patterns at approximately equal frequencies. PB800 also exhibited a sixless phenotype, in which ray six is morphological transformed and fused with ray 4, at an approximately 1% frequency. In C. elegans this phenotype results at high frequency from mutations
mab-18 and mab 21 and at low frequency from haploinsufficiency of
egl-5 . Moreover, altered levels
egl-5 and
mab-5 expression can mimic the canonical C. briggsae ray pattern. Thus, ray pattern variation in C. briggsae may result from allelic variation in C. briggsae HOM-C genes. To test this hypothesis, PCR product length polymorphisms have been identified in the C. briggsae homolog of
egl-5 ( Cb_egl-5 ). These Cb_egl-5 polymorphisms will be tested for linkage to variation in ray pattern. If linkage is detected, additional experiments will be conducted to map the C. briggsae ray pattern variants to a single locus.