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expression ( Fig. 8G ) can be enhanced using complex arrays ( Kelly et al., 1997 ),
integrated low copy number microparticle bombardment transformation strategies
( Praitis et al., 2001 ), or Mos1 transposon single copy insertion (MosSCI) based
methods ( Robert and Bessereau, 2010 ). Reliable expression of sperm fusion proteins
has not yet been achieved. For a more in-depth discussion of protein localization
studies see the chapter by Hutter.
XIII. Future Prospects/Issues
This chapter attempts to provide a useful overview of how C. elegans can be used
as a model system for addressing questions of fertility. A clear understanding of any
biological process as complex as fertilization is impossible without a complete
inventory of its cellular and molecular components; consequently, each new gene
identified adds significantly to our knowledge and insight
into fertilization
mechanisms.
Although much has already been elucidated via C. elegans fertility research,
many fundamental questions remain unanswered. For example, how are the
events surrounding fertilization (e.g., meiotic maturation, ovulation, fertiliza-
tion, sperm migratory behavior, and the nature of signaling events) coordinated?
What is the mechanism of the block to polyspermy? How are the events of
fertilization and egg activation related to early development and patterning of
the embryo?
The fundamental question of what happens to sperm and oocyte fertility
proteins during the physical joining of the gametes is being actively studied
in our laboratories. The investigation of this deceptively simple question has
proven challenging since there are at most only two recently fertilized oocytes
present in any hermaphrodite at one time, and only those in the very earliest
stage of postfertilization meiosis are informative (A. Richmond and D. Shakes,
unpublished data). Although difficult, such studies are feasible and will be
ultimately useful in analyzing the interactions between sperm and oocyte fer-
tility proteins.
Exciting progress continues to be made and the field is still developing new
experimental tools. Calcium imaging, for instance, has been applied to C. elegans
sperm and oocytes ( Samuel et al., 2001; Xu and Sternberg, 2003 ). Using Calcium
Green-1 dextran as an indicator, an increase in cytoplasmic calcium is observed at
the same time as fertilization, but the trigger for the calcium release and its func-
tional consequence are unknown ( Samuel et al., 2001 ). Voltage-sensitive reagents or
other physiological approaches could be used to test whether the fast block to
polyspermy is dependent on depolarization of the oocyte plasma membrane
( Yanagimachi, 1994 ) as in other organisms. New transgenic approaches are helping
to overcome the germ line expression repression that currently hampers research in
this area ( Robert and Bessereau, 2010 ). In vitro fertilization systems in C. elegans
could be useful for analyzing the function of newly discovered molecules; however,
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