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move towards the uterus and are ovulated into the spermatheca, where they immediately
contact multiple spermatozoa. Blocks to polyspermy exist, as only a single sperm
fertilizes each oocyte (
Parry et al., 2009
). The coordination of events leading to
sperm/oocyte contact ensures highly efficient spermutilization as virtually all functional
sperm fertilize oocytes (
Kadandale and Singson, 2004; Ward and Carrel, 1979
). The
hermaphrodite
'
s own sperm can be supplemented by mating to males; male sperm are
deposited in the uterus and immediately travel to the spermatheca to await oocyte
passage. A sperm-sensing mechanism ensures that metabolically costly oocytes are
not wasted; when hermaphrodites lack sperm, they ovulate at a very low basal level.
Conversely, the presence of sperm within the hermaphrodite spermatheca causes a
dramatic increase in ovulation rate (
McCarter et al., 1999; Miller et al., 2001
). After
fertilization, the zygote secretes a multilayered eggshell and begins embryonic devel-
opment. Eggs then pass through the uterus and are laid before hatching (
Fig. 2
).
C. elegans offers several advantages over other model systems for studying
fertilization. Although its amoeboid sperm possess neither a flagellum nor an acro-
some (
Fig. 3
), these sperm successfully perform the same tasks required of all
spermatozoa (e.g., migration to the fertilization site, species-specific oocyte recog-
nition, fusion). In addition, the events of fertilization can be directly observed in
living animals through the worm
'
s transparent cuticle (
McCarter et al., 1999
). It is
also possible to isolate large quantities of sperm and oocytes, though this is more
challenging to do than in some other model systems (
Aroian et al., 1997; L
'
Hernault
and Roberts, 1995; Miller, 2006
). Fertilization studies in C. elegans routinely use
molecular and genetic tools that are unavailable or difficult to use in other systems.
The complete sequencing of the worm genome and the availability of microarrays
greatly simplifies the identification and analysis of genes required for fertility
(
Singson, 2001; Singson et al., 2008
). Perhaps the greatest advantage of C. elegans
is the ease with which one can perform forward genetics to screen for fertilization-
defective mutants (discussed below). Such screens have identified many of these
mutants, which may be classified broadly into those mutations affecting sperm (spe
or fer mutants, for spermatogenesis or fertilization defective) and those affecting
eggs/oocytes (egg mutants). Note that the fer designation has been discontinued and
all new sperm development or function mutants are now given the spe designation.
Although the majority of characterized spe/fer mutations affect sperm develop-
ment, a subset of these mutations specifically affects sperm function (i.e., fertiliza-
tion). spe-9 class mutants, for example, produce sperm that are unable to fertilize
oocytes despite exhibiting normal morphology, motility, and gamete contact
(
Chatterjee et al., 2005; Kroft et al., 2005; L
'
Hernault et al., 1988; Singson et al.,
1998; Xu and Sternberg, 2003
). Recent studies have also uncovered egg mutations
that specifically influence fertilization and/or egg activation (
Kadandale et al.,
2005b
). A partial listing of characterized genes required for fertilization is given
in
Table I
. Many of the experimental tools and techniques discussed in this chapter
were developed for the study of these genes.
C. elegans also enables the evolutionary assessment of fertilization molecules and
can help elucidate major molecular themes. For example, EGF-repeat-containing
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