The sexual nature of the eukaryote genome.

This paper supports a previous conjecture that the sexual cycle of eukaryotes arose from the infection of cells by genome parasites. The finding are as follows. (1) In prokaryotes, conjugative plasmids ensure their own spread by directing partial cell fusion. (2) Conjugative plasmids permit gene transfer between prokaryotes and eukaryotes, and can be integrated into the eukaryote genome. (3) Genes can be transferred between unrelated eukaryotes, and between different genomes within eukaryotic cells. (4) Elements such as transposons and retroviruses evolve as parasites of the eukaryote sexual system. (5) The mating-type genes of bipolar fungi are idiomorphic: alternative genes directing sexual specificity are dissimilar and non-homologous. It is argued that they arose as parasitic elements directing cell fusion, which have become integrated into the genome. (6) Mating-type idiomorphs in different taxa may be dissimilar, reflecting the independent acquisition of different infectious elements by different eukaryote lineages. (7) Sexual competence is rapidly lost through mutation accumulation or antagonistic pleiotropy during vegetative proliferation, so that many lineages are sexually sterile. (8) In multipolar systems, novel mating-type alleles arising by mutation spread in a parasite-like manner by virtue of their access to lineages which have become sexually sterile. (9) It is suggested that centromeres arise as devices to enable low copy number plasmids to persist. (10) Crossing over is favored if it enables the genes that direct it to unlink themselves from inferior genomes, or from the consequences of their own transposition.(ABSTRACT TRUNCATED AT 250 WORDS)