Changes in chloroplast genome composition and recombination during the maturation of zygospores of Chlamydomonas reinhardtii.

In crosses of the unicellular green alga Chlamydomonas reinhardtii, the chloroplast genes are normally transmitted exclusively by the maternal parent to zygospore progeny. However, transmission of the paternal chloroplast alleles can be increased markedly by certain pretreatments of the maternal parent prior to mating. As zygospores age prior to induction of meiosis, they display decreased biparental transmission of chloroplast alleles and increased transmission of chloroplast alleles from only the maternal or paternal parent. In this report, chloroplast genome composition of biparental zygospores is shown to change in several ways during zygospore maturation. Allelic ratios of chloroplast genes within biparental zygospore clones become maternally or paternally skewed as the zygospores age, cotransmission of chloroplast alleles is reduced, and recombination increases, resulting in an expansion of genetic map distances between chloroplast markers used in this cross. The recovery of unequal frequencies of zygospore progeny expressing reciprocal recombinant genotypes confirms and extends other reports of the predominance of nonreciprocal recombination in organelle genetic systems.