A study of interconversion of mating systems in desmids as inferred from selfing in Closterium ehrenbergii

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Most if not all clonal isolates from natural populations of the green alga Closterium ehrenbergii show heterothallism which is controlled by a pair of alleles at the mating-type(mt)locus, we obtained, however, selfing clones in progeny of a cross involving the mt^- J5-48-2 clone which was interpreted as having unique chromosomal rearrangements. Although derived from heterothallic parents, these selfing clones could form zygotes between cells of the same single clone as do homothallic strains. All the selfing clones could be considered basically as mt^-, because they were crossed well with any mt^+ clone and turned out to inherit the mt^- allele from the J5-48-2 parent. Nonetheless, they could be crossed with any mt^- clone as well. Crossed zygotes yielded high-viability progeny, but selfed zygotes gave rise to extremely low-viability progeny. Selfing clones appeared in progeny when a selfing parent was selfed or crossed with a mi^+ clone but not with a mt^- clone. By spliting and recloning a number of pairs of conjugating gametangial cells, we confirmed that a selfing clone produce both mt^+ and mt^- cells, in addition to cells continuing selfing, during vegetative growth by mitotic cell divisions. We discuss the possibility that selfing in C. ehrenbergii may occur by unidirectional gene conversion from mt^- to mt^<+. >