3.11.2: Male and female

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In syngamy, there are always two partners. They could be superficially almost identical—only genotypes and some surface chemistry differ. Two dangers are here: to mate with same genotype (then why to mate?) or mate with other species (which could result in broken genotype). To improve partner recognition, there are many mechanisms.

One is based simply on size. If one is bigger, then smaller one is presumably the good partner. From this point, smaller cell is called male, and bigger called female.

Now, females could invest in storage (and bigger size) whereas males invest in numbers. This strategy will dramatically improve fertilization and also allows to select better males. It results in big, non-motile female cells and small, fast-moving, numerous male cells. Here females called oocytes (or egg cells) and males—spermatozoa.

The ultimate step could be non-motile males too, but this is not frequent because they will need the external help for the fertilization. These non-motile male cells (spermatia) exist in red algae, sponges, crustaceans and flowering plants.