parasexual

Parasexual refers to a mode of genetic recombination observed in certain fungi and protists that occurs without meiosis or conventional sexual reproduction. In a parasexual cycle, genetic exchange is achieved through cell fusion and subsequent genetic rearrangements within a shared cytoplasm, followed by random loss or inactivation of chromosomes to restore a haploid or near-haploid state.

In parasexual cycles, cells may first form a diploid or tetraploid state through cell fusion (plasmogamy and karyogamy), creating a nucleus or heterokaryon that contains multiple chromosome sets. Mitotic recombination and crossing over can occur during mitotic divisions, producing new genetic combinations. Later, genome reductions by chromosome loss return the lineage to a haploid or near-haploid state, yielding recombinant haploids or aneuploids. Importantly, this cycle does not involve meiosis or the production of sexual spores.

Parasexual cycles have been observed in several fungal species, notably Candida albicans and Aspergillus nidulans, where sexual cycles are absent or rare. In laboratory settings, compatible strains can be fused to form a tetraploid, after which random chromosome loss yields recombinant haploids. These recombinants can be isolated and analyzed to study gene linkage, chromosome behavior, and recombination without a meiotic sexual cycle.

Significance and characteristics: parasexuality provides a mechanism of genetic variation in organisms lacking conventional sexuality, serving as a tool for genetic mapping and genome studies. It can produce stable or semi-stable variants but often results in aneuploid progeny with altered fitness. The process is generally less efficient and less predictable than meiosis and does not generate standard sexual spores.

The term parasexual derives from para-, meaning alongside, reflecting a process that operates alongside traditional sexual mechanisms.