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and yield results comparable to the more labor-intensive direct observations. In
either method, age-matched controls are absolutely essential because the rate-lim-
iting factor for ovulation rate changes with age; for young adults, it is oocyte growth
(
Miller et al., 2003
) while for older animals it is sperm. As with brood assessments,
relative differences are more important than absolute numbers, and appropriate
statistical analysis is mandatory.
Ovulation rates can be measured in either self-crossed or male-mated hermaph-
rodites. Self-crossed hermaphrodites are easier to use but if they become sperm-
depleted during the course of the assay, their ovulation rates will decrease over time
and result in lower overall averages unrelated to the mutant-of-interest. To avoid
sperm-depletion effects, mutant hermaphrodites may be plated with wild-type males
to maintain nonbasal ovulation rates for the duration of the assay. Conversely, the
ability of mutant sperm to induce ovulation can be assessed by measuring the
ovulation rates of fem or fog ''females'' crossed with mutant males.
Many ovulation-defective mutants exhibit a secondary phenotype whereby
oocytes undergo multiple rounds of DNA synthesis to become polyploid
(
Iwasaki et al., 1996
). The location of these endomitotic (EMO) oocytes offers clues
to the nature of the mutation; EMO oocytes in the gonad arm suggest spermathecal
and/or ovulation defects while EMO oocytes in the uterus suggest defects in fertil-
ization itself or in egg activation following sperm entry. Note that in the absence of
sperm, wild-type hermaphrodites also produce EMO oocytes in their uterus (
Ward
and Carrel, 1979
).
VI. Spermatogenesis
In C. elegans, gametogenesis and early spermatogenesis occur in a linear pro-
gression along the length of the tube-like gonad (
Fig. 6
, but also see
Fig. 1
). After
Fig. 6
Spermatogenesis in C. elegans. Major developmental stages are indicated.
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