SEXUAL REPRODUCTION in Chlamydomonas reinhardtii, a heterothallic, unicellular green alga, yields two types of zygotes: the thick-walled, dormant zygospore (meiotic), and less frequently, the vegetative zygote which divides mitotically soon after initial gamete fusion to produce stable diploid progeny1,2. Although vegetative zygotes account for less than 5% of the products of gamete fusion, their diploid progeny can be easily selected on minimal medium by crossing haploid strains which carry complementing auxotrophic mendelian mutations2. I report here that differences in chloroplast inheritance observed between meiotic and mitotic zygotes of Chlamydomonas are correlated to the timing, rather than the nature, of their first divisions. Environmental conditions which delay the first post-mating division of vegetative zygotes result in uniparental inheritance while favourable post-mating environments promote biparental inheritance of chloroplast genes.
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