|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.
|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.