A human egg cell, the corona radiata is on the outer surface, the zona pellucida is the clear ring behind it.
Oocyte is the name for an immature egg cell (ova, in animals), an oocyte only becomes an egg cell finally when it is fertilized.
The oocyte and the spermatozoa (sperm cell) are the sex cells of organisms which reproduce sexually, that is by combining the genetic material of two different organisms. Yet, the very earliest cellular organisms must have reproduced asexually, so why does the vast majority of complex organisms produce sexually exclusively? What caused this change, and why did it proliferate?
The oocyte and the spermatozoa (sperm cell) are the sex cells of organisms which reproduce sexually, that is by combining the genetic material of two different organisms. Yet, the very earliest cellular organisms must have reproduced asexually, so why does the vast majority of complex organisms produce sexually exclusively? What caused this change, and why did it proliferate?
There
is much speculation on why so many organisms switched from asexual to sexual reproduction. It is likely that sexual
reproduction arose in an ancestor of the eukaryotes (organisms with more complex cells), deep in the
Precambrian. But sexual reproduction has an immediate flaw,
only half the population are capable of producing offspring physically, the
females. An asexual population would quickly out-compete a sexual one
purely by virtue of being able to grow at twice the rate of a sexual
one. This is known as the “two-fold cost of sex”. However, this overlooks
the role of the male. Males provide genetic diversity to a
particular population, as when the sperm and ovum haploid genomes
recombine in fertilization to form a zygote with a diploid genome, and therefore two
copies of the same gene from each parent. In recombination, genetic
“mistakes” in the sequence of base pairs should be corrected. Hybrids between two populations of the same species tend to be
fitter genetically, a process known as heterosis (this is not always
the case in all hybridizations, however). Therefore, an organism produced
sexually may be more resistant to disease than asexually produced
organisms. This is probably why almost all complex organisms (except very few, like the New Mexico whiptail lizard, which can also be created by hybridizing two other whiptail species) reproduce sexual, as they need to compete in a more dynamic ecology, so need the fast acquisition of advantageous traits in the whole population that only sexual reproduction.
Isogamy and Anisogamy
Some
of the more primitive single-celled organisms reproduced by fission,
which is regular cell division into genetically identical offspring
cells. However, most single celled organisms swap genes, occasionally, through
genetic recombination or the transfer of a nucleus mostly with organisms from a separate population, which has
accumulated different genetic mutations to it's population. This is probably how
sexual reproduction arose in the earliest organisms. Eventually
specific cells began to specialize, by carrying the haploid genome of
the organism, so it would combine with the haploid genome of another
organism to create a diploid organism. This is this basis of sexual
reproduction.
The
gametes in these early sexual organisms would have appeared the same, only the haploid genome would be different between the two. This is known as isogamy,
where each gamete has the same morphology. The gametes in some
organisms began to specialize further, into anisogamy, where one
gamete is larger than the other. One modern partially isogamous
species is the green algae Volvox. In a study, colonies where
all the gametes were small (isogamy) produced a smaller population,
whereas anisogamous populations were larger. This shows that when
there is one large gamete, which has a larger energy reserves (yolk granules in the cytoplasm) but
is slower, and one small gamete, which can move faster but does not have
large energy reserves, population sizes are larger. Anisogamy
means that one gamete, the oocyte, can specialize in providing a
larger energy supply for the zygote after fertilization and the
other, the spermatozoa can specialize in being fast and numerous. This is a particular type of anisogamy known as oogamy.
Due
to the large tax on energy resources for the female there is a great
difference between ovum and sperm number, there are around 400 ovum
ovulated by a woman who has reached menopause compared to 200-500
million spermatozoa per ejaculate for human males, and up to 8
billion per million per ejaculate for domesticated pigs (who have
been breed to be as fertile as possible).
Egg Laying
The
first vertebrates to have ova were reptiles, as when on land they
could not reproduce by spawning. In reptiles, and later in birds, the
ova once fertilized forms protective layers (the egg shell) and is
passed out through the oviduct. In some of these species, the embryo is then
kept warm (incubated) by mostly the mother (the father and mother
Whooping Crane alternatively incubate the egg). This is known as
ovipartity, and the embryo receives nutrition only from the nutritive
yolk, meaning than in oviparous animals, more energy is needed to
create the ovum than in viviparous animals (who have live young). But
after fertilisation, there is less need for parental care, therefore enabling more offspring to be produced. The
offspring, due to lack of long-duration parental care, have a higher
mortality rate, but more are produced in relation to the amount of
energy the parents lose in reproduction. Only a small number of
mammalian species produce eggs (the monotremes) including the duck-billed
platypus, which are descended from the some of the earliest mammals and hence having features common and almost unique
to both mammals and reptiles.
The Zona Pellucida
The oocyte has multiple barrier to stop polyspermy (multiple sperm entering
the oocyte, therefore producing a cytoplast with around double the normal number of chromosomes for the species, which would be unable to survive).
Polyspermy may arise from having sperm that is too efficient at
surviving and fertilising the oocyte, therefore a male with very
efficient sperm may not pass on it's genes due to his offspring's
inability to survive.
Sperm
that is more efficient at fertilising an oocyte does not mean that it is
beneficial to the female (for reasons as seen above) as this sperm
may not have the best genetic material to pass onto successive
generations. If mammals and reptiles evolved using spawning as a reproductive
method, there would be all sorts of genetic problems as the only
“screening process” would be that the sperm would be from an
animal that had reached puberty. This gives rise to a very complex genital system in polygamous
species, such as that seen in the Pekin Duck.
The female Pekin Duck has a maze-like vagina to be able to “pick”
the best sperm for reproduction. To counter this the male has a
“corkscrew”-like penis and only the males with the precisely
shaped penis for that one female's vagina will fertilise the oocyte.
The
first barrier to the sperm is the zona pellucida, then the plasma
membrane, which also protect the fertilized ova on it's journey through the
oviduct (or in the water in fish and amphibians). The zona pellucida
is an essential feature, if it is not present the female will be
infertile. It stops sperm from other, genetically incompatible
species from binding and also emits chemicals that guide the sperm
cells towards the oocyte. In ovipartial species, when the nucleus of the
sperm and oocyte fuse, the zona pellucida signals for a release of
calcium ions which eventually transform into the egg shell. For fertilization to occur the nucleus of the sperm needs to bind with
the zona pellcida and “bore through” it, fusing in the oocyte's cytoplasm. After fertilisation the cortical reaction is triggered,
which causes the plasma membrane to fuse and become impermeable to
sperm. Without the zona pellucida, gametes from incompatible species can fuse, though the offspring are not
viable.
Genes
encoding on the surface of gametes are some of the most rapidly
evolving genes, even faster than those responsible for immune
responses, which have to adapt quickly due to microbe attacks. If the
oocyte is "under pressure" to reduce polyspermy, the proteins on the
surface of the oocyte will change, so that fewer sperm are compatible
with the oocyte, meaning there is a smaller probability of polyspermy.
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