Mating

In some terrestrial arthropods, including insects representing basal (primitive) phylogenetic clades, the male deposits spermatozoa on the substrate, sometimes stored within a special structure.

In advanced groups of insects, the male uses its aedeagus, a structure formed from the terminal segments of the abdomen, to deposit sperm directly (though sometimes in a capsule called a "spermatophore") into the female's reproductive tract.

Vertebrates reproduce with internal fertilization through cloacal copulation (in reptiles, some fish, and most birds)[3] or penile-vaginal penetration and ejaculation of semen (in mammals).

Based on a phylogenetic analysis, Dacks and Roger[9] proposed that facultative sex was present in the common ancestor of all eukaryotes.

A principal reason for this view was that mating and sex appeared to be lacking in certain pathogenic protists whose ancestors branched off early from the eukaryotic family tree.

To cite one example, the common intestinal parasite Giardia intestinalis was once considered to be a descendant of a protist lineage that predated the emergence of meiosis and sex.

However, G. intestinalis was recently found to have a core set of genes that function in meiosis and that are widely present among sexual eukaryotes.

[11] Other protists for which evidence of mating and sexual reproduction has recently been described are parasitic protozoa of the genus Leishmania,[12] Trichomonas vaginalis,[13] and acanthamoeba.