Transmission and recombination of chloroplast genes in asexual crosses of Chlamydomonas reinhardii

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Abstract
Chlamydomonas reinhardii diploids homozygous for the plus mating-type (mt +) allele were constructed via polyethylene glycol (PEG)-induced cell fusion to investigate the transmission of chloroplast genes. We used two methods to determine whether the fusion products (PEG diploids) had inherited chloroplast markers uniparentally or biparentally. One method (multiple clone analysis) was found to markedly improve the detection of biparental transmission. With this method the frequency of biparental PEG-induced diploids was comparable to that seen in sexual diploids. Multiple clone analysis also demonstrated that fusion products which showed biparental inheritance were an extremely heterogeneous group. In some crosses, pre-treatment of one parent with isolated flagella from cells of the opposite mating type was used. This pre-treatment has been reported to promote chloroplast gene transmission from the mt + parent in diploids produced by PEG fusion (Adams 1982). In our crosses flagellar pretreatment had no significant effect on chloroplast gene transmission or recombination frequences. A significant bias was found for the date a cross was performed, but neither parent preferentially transmitted its alleles in all crosses. This indicates the necessity of comparing data from treated and control crosses done on the same day. We conclude from our data that separate hypotheses are not required to explain biparental inheritance in sexual versus PEG-induced diploids. The absence of biased parental allelic ratios with PEG-induced biparental diploid clones underscores their usefulness for the study of biparental gene transmission.