Animals carrying the mutation
unc-93(
e1500) are defective in egg-laying, motility, and spicule retraction and display the 'rubber band' phenotype, in which a touch to the head elicits contraction of the body with no backward movement. All of these abnormalities are suppressed by loss-of-function alleles of
sup-9 .
unc-93 and
sup-9 encode transmembrane proteins that, along with the products of
sup-10 ,
sup-11 , and
sup-18 , regulate muscle contraction.
sup-9 encodes a presumptive potassium channel. Two novel ENU-induced alleles of
sup-9 were isolated as suppressors of the motility defect of
unc-93(
e1500) mutants. Like
sup-9 null alleles, the novel alleles
sup-9(
lr164) and
sup-9(
lr165) are recessive suppressors of
unc-93(
e1500) homozygotes, restoring motility and egg-laying to wild-type levels. However, unlike previously characterized
sup-9 alleles,
sup-9(
lr164) and
sup-9(
r165) are dominant suppressors of partially uncoordinated
unc-93(
e1500) heterozygotes, suggesting that these alleles encode a SUP-9 protein with an altered or dominant-negative activity. Also, while homozygous null alleles of
sup-9 restore wild-type spicule function to
unc-93(
e1500) males,
sup-9(
lr164) and
sup-9(
lr165) fail to do so. In addition, these novel suppressor alleles complement
sup-9(
n180) , which we found to have a mutation in the first splice-acceptor sequence, but they fail to complement a nonsense allele,
sup-9(
n1913) . We are currently investigating the mechanism of
sup-9(
n180) complementation and are testing other
sup-9 alleles for similar complementation. Both
sup-9(
lr164) and
sup-9(
lr165) are T-to-G transversion mutations.
sup-9(
lr165) results in an L-to-R change in a putative cytoplasmic loop between the second and third membrane-spanning domains of SUP-9, while
sup-9(
lr164) results in an M-to-R change near the end of the third membrane-spanning domain. Strains containing both
unc-93(
e1500) and either
sup-9(
lr164) or
sup-9(
lr165) exhibit a severe male-mating defect in spite of their normal motility. This defect is dependent on the presence of
unc-93(
e1500) ; males containing only
sup-9(
lr165) mate as well as wild-type males while
sup-9(
lr164) males have slightly lower male mating efficiency.
unc-93(
e1500) males are also unable to mate, but the underlying cause of this defect has not been elucidated. Direct observation and use of the male leaving assay of Lipton and Emmons (1999, WBG 16(1):56) suggest that the male-mating defect of suppressed strains is primarily behavioral. In leaving assays, single males are placed in a small spot of food with five paralyzed hermaphrodites. The rate at which males leave a circle of 7 cm diameter is determined. Wild-type males tend to stay with the hermaphrodites, and to leave the circle when hermaphrodites are absent. We found that
sup-9(
lr164 or
lr165);
unc-93(
e1500) males have a higher rate of leaving the circle in the presence of hermaphrodites than do wild-type males. Male behavior in the absence of hermaphrodites is unaffected. Mating experiments demonstrate that males containing
unc-93(
e1500) and certain other
sup-9 missense alleles also have a decreased ability to mate. Experiments are underway to determine whether the male-mating defects are a consequence of interactions between the
sup-9 lesions and
unc-93(
e1500) or of an incomplete suppression of a heretofore uncharacterized male behavioral defect conferred by
unc-93(
e1500) .