In C. elegans , hermaphrodites make both sperm and oocytes within its gonad, all
ID: 68390 • Letter: I
Question
In C. elegans, hermaphrodites make both sperm and oocytes within its gonad, allowing it to be self-fertile (much in the same way as Mendel’s pea plants). There is a homozygous mutant (fem-3(lf)/fem-3(lf)) that fails to make sperm (makes oocytes only) and another homozygous mutant (fbf(lf)/fbf(lf)) that fails to make oocytes (makes sperm only). These genes, fem-3 and fbf, are hypothesized to function in the same regulatory pathway. Interestingly, the double mutant fails to make sperm (makes oocytes only). (NOTE: both null alleles are recessive to WT).
a) Which gene is epistatic?
b) Which gene likely functions upstream?
c) Draw a simple regulatory pathway to explain the data.
d) Later you identify dominant alleles of the fem-3 gene. All dominant alleles are gain-of-function alleles and map to the 3’ UTR! As gain-of-function alleles, they have a distinct mutant phenotype. Instead of producing only oocytes, animals that are fem-3(gf)/+ or fem-3(gf)/fem-3(gf) produce sperm only. With this added bit of information, speculate how fbf might function (state a specific hypothesis).
Explanation / Answer
a. An epistataic gene determines if a trait shall be expressed or not. Here, the fbf gene is epistatic as it clearly indicates this gene product may be essential to repress the production of oocytes.
c. The fbf gene is likely to function upstream
d. fbf might bind to 3' upstream region of fem-3 mRNA and inhibit it at translational level, preventing it from producing oocytes.