Chloroplast gene transmission in Chlamydomonas reinhardtii: A random choice model

Chloroplast genes in Chlamydomonas reinhardtii are inherited uniparentally. In a laboratory cross the majority (>95%) of the zygotes transmit to the meiotic progeny only those chloroplast genes donated by the maternal parent. This occurs even though the parents are isogamous and the chloroplasts from the two parents fuse shortly after mating. Uniparental inheritance of the chloroplast genes can be altered by several methods. If maternal gametes are irradiated with ultraviolet light prior to mating, the proportion of zygotes transmitting chloroplast genes from the paternal parent rises dramatically. In this paper I examine in detail the effects of uv irradiation on both maternal and paternal gametes and the effect of photore-activation following the uv irradiation. The effect of uv irradiation can be largely reversed by photoreactivation, and both the starting time and the intensity of the photoreactivating light used are found to be critical. Examination of the frequencies of the different zygote types obtained with respect to chloroplast gene transmission following uv treatment of the maternal gametes shows that they fit a hypergeometric distribution, in which choices are made from a population without replacement. By rearranging the basic hypergeometric equation I was able to estimate that the choice is made from a population of 27 maternal and 2 paternal units of gene transmission. These units probably contain more than one genome each, since their number is much lower than the estimated number of genomes per cell. My model explains both the observed distribution of zygote types and the bias in favor of maternal alleles found in the progeny of a biparental zygote, and may have a wider application to other organelle genetic systems. I also suggest that the extreme degree of uniparental inheritance of chloroplast genes in Chlamydomonas found in the laboratory may not be seen in nature.