Epigenetics of Reproduction in Animals - Building the Most Complex Structure on Earth - page 100

result of meiosis of the primary oocyte is the production of one mature egg and three
polar bodies, which undergo apoptosis and are eventually reabsorbed by the organ-
ism. The oocyte grows into a mature egg cell and is released, ready for fertilization
by a sperm cell.
Meiotic division during spermatogenesis is similar to oogenesis. The main differ-
ence is that while the meiosis of a primary oocyte results in the production of only
one gamete and three polar bodies, the primary spermatocyte produces four sperm
cells via meiotic divisions.
Hermaphroditism in Metazoans
Most metazoans have separate sexes, while most plants are hermaphroditic. The
above discussion on sexual reproduction was about animals that have separate
sexes. However, a considerable number of metazoans are hermaphroditic, display-
ing male and female breeding modes. There are two main forms of hermaphroditism.
Simultaneous hermaphroditism is when the same organism has both the male and
female sex organs and produces both types of gametes. Sequential hermaphroditism
means that an organism switches from its inborn sex to the opposite sex, a develop-
ment observed primarily in certain fish and gastropods. Many of these hermaphro-
ditic species can reproduce asexually, through their own gametes, or sexually, when
their eggs are fertilized by sperm cells from other conspecific individuals.
The most plausible hypothesis on the evolution of simultaneous hermaphrodit-
ism is the limited availability of mating partners. Accordingly, the low mobility and
low population density favor the evolution of hermaphroditism, which offers selec-
tive advantages such as maximized fitness deriving from coupling male and female
functions in a single organism, a higher likelihood of meeting a partner because all
individuals are potential mates, and, in the absence of mating partners, the option
of producing offspring by self-fertilization ( Puurtinen and Kaitala, 2002 ). Statistical
studies show that, besides low mobility and low population density, another impor-
tant factor in driving the evolution of hermaphroditism is mate-search efficiency
( Eppley and Jesson, 2008; Puurtinen and Kaitala, 2002 ). “When mate search is effi-
cient, disruptive frequency-dependent selection on time allocation to mate search
leads to the evolution of searching and nonsearching phenotypes and, ultimately, to
the evolution of males and females.” ( Puurtinen and Kaitala, 2002 ). Returning from
hermaphroditism to separate sexes is thought to be easier than the evolution of her-
maphroditism because giving up a sex function might be easier than assuming one
( Charnov, 1982 ).
So far, we have dealt with factors that may stimulate the evolution of hermaph-
roditism or separate sexes, but not with the mechanisms that produce this complex
transformation. The reason is no secret; all the studies on hermaphroditism deal
with the selection of the new trait instead of its evolution or emergence. However,
there is compelling evidence of the mechanism of sequential hermaphroditism. It is
about cases of sequential hermaphroditism described in at least 25 families of fish
( Devlin and Nagahama, 2002 ). The fact that this incredible change occurs during