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| Cromosomas X e Y |
Sexual reproduction is the most
common form of reproduction among eukaryotes, including multi-cellular living
beings. After billions of years of asexual reproduction among prokaryotes, who
share genetic information by exchanging plasmids (small DNA fragments), a new
type of reproduction suddenly appeared. As it was successful, we must assume
that it must have provided some advantage over the alternative procedure.
Sexual reproduction can be defined
as the alternation between two life cycles for the same type of organism:
- Haploid cycle: each cell has a single copy of every chromosome.
- Diploid cycle: each cell has two copies of every chromosome.
In eukaryotes, the haploid cycle is
always unicellular; the diploid cycle may be unicellular (in unicellular
eukaryotes) or multi-cellular (in multi-cellular eukaryotes). The individuals in
the haploid cycle are called gametes.
How was this alternation
established? It could have been caused by an alternation between two different
environments. Haploid and diploid cells do not have the same properties. For
example, diploid cells are more voluminous, having many duplicate organelles,
so that the ratio of surface to volume is usually larger than in haploid cells (about
1.25 times). As the absorption of nutrients by the cells depends on their surface,
haploids tend to grow faster than diploids.
- Endo-mitosis: a single individual goes from the haploid to the diploid cycle
without merging its genetic endowment with another individual.
- Singamia (fertilization): the union of two different haploid individuals to
form a single diploid individual.
- Simple meiosis: a form of cell division that goes from one diploid individual to
two haploid individuals.
- Double meiosis: a form of cell division where an endo-mitosis gives rise to a tetraploid cell which then divides into four haploid cells. Crossing-over between homologous chromosomes is particularly easy with this system.
The most extended type of sexual
reproduction in multi-cellular eukaryotes consists of a singamia, the beginning
of the diploid cycle, followed by a double meiosis, that gives rise to the haploid
cycle. Other combinations in unicellular eukaryotes include endo-mitosis
followed by simple meiosis, or singamia followed by simple meiosis. Some of
them might have served as intermediate stages for the onset of the typical sexual
reproduction, but the exact mechanisms, and the reasons why natural selection
has been able to encourage each of the intermediate steps, are unknown.
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| Possible origin of anisogamy |
Another outstanding issue is the
origin of anisogamy: why the haploid gametes have diversified into two
different types, usually called male and female. Even isogamous unicellular
eukaryotes (those with similarly shaped gametes) have two morphologically
indistinguishable types of gametes (+/-). When two gametes fuse, they must
belong to different types, they would never join if they belong to the same
type. It has been suggested that each type of gamete could generate a
complementary protein (a male and a female protein) that must assemble together
during the fusion of the two cells. Each cell would have initially produced the
two proteins, so being able to merge with an identical cell, but then the two
genes were split in different individuals, so that each type of cell could only
be assembled with the other. We don’t know why the separate version would have
had a selective advantage over the ambivalent. Perhaps the differentiation in two
types of gametes had the advantage that cellular organelles (such as
mitochondria) would be inherited from only one gamete, thus avoiding intragenomic
conflicts, but this explanation is not certain.
Images adapted from the book "The major transitions in evolution" by John Maynard Smith and Eörs Szathmáry, OUP 1997.
Manuel Alfonseca
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