Chloroplast inheritance in triploid and tetraploid vegetative zygotes of Chlamydomonas reinhardtii

Abstract

Triploid and tetraploid mitotic (vegetative) zygotes of Chlamydomonas reinhardtii have been isolated from crosses between complementing haploid and/or diploid mutant gametes homozygous or heterozygous for the mating-type (mt) locus and the transmission of chloroplast genes conferring resistance to antibiotics has been analyzed in their mitotic progeny. In control crosses mt + x mt −, the transmission of chloroplast genes in vegetative zygotes was most often biparental (71–86%) and more frequently maternal (13–28%) than paternal (1–5%). In crosses mt +/mt + x mt −, mt +/mt + x mt −/mt − or mt +/mt + x mt +/mt − , the chloroplast paternal gene was also poorly transmitted but the transmission was more often maternal than biparental. In crosses mt + x mt −/mt − and mt + x mt +/mt −, the mode of transmission was strongly dependent on the particular chloroplast marker carried by one or the other parent. Analysis of frequencies of markers segregating in biparental zygote clones revealed a similar marker effect in all crosses. The observation that mt +/mt − and mt −/mt − gametes (phenotypically mt − for sex expression) behave similarly in the control of chloroplast inheritance indicates that mt + gene function is not expressed in those mt +/mt − diploid gametes which mated with mt + or mt +/mt + gametes to yield triploid or tetraploid vegetative zygotes. This strongly contrasts with what is observed in crosses leading to meiotic zygotes (mt + dominant over mt − with regard to chloroplast inheritance). Thus triploid and tetraploid vegetative zygotes differ from the meiotic zygospores not only by their evolution toward mitotic division instead of maturation but also in terms of mt control over chloroplast heredity. In this regard, diploid gametes destined to form vegetative zygotes differ from those which will mate to produce zygospores, perhaps in a manner similar to comparable differences within haploid gamete population.