Patterns of organellar and nuclear inheritance among progeny of two geographically isolated strains of Volvox carteri.

Strains of Volvox carteri forma nagariensis derived from Japanese and Indian isolates ("J" and "I" strains, respectively) exhibited length differences (RFLPs) for approximately 90% of the restriction fragments detected by hybridization with a variety of unique-sequence, small-gene-family and repetitive-element probes, including heterologous probes of chloroplast and mitochondrial origin. Extensive post-zygotic mortality was observed among the zygotes produced by crossing J and I strains, suggesting some form of genetic incompatability between them. Most of the viable progeny exhibited recombinant patterns of nuclear inheritance and maternal inheritance of mitochondrial and chloroplast markers. However, many progeny exhibited exclusively uniparental (usually maternal, but in one case paternal) inheritance of both nuclear and organellar markers. Some of these non-recombinant individuals may be derived from "parthenospores" (dormant asexual cells resembling zygospores). Others may be a result of "pseudogamy," in which one of the parental pronuclei is excluded from the zygote, followed by selective exclusion of both the mitochondrial and the chloroplast genomes derived from that same parent. When segregation patterns for 44 nuclear markers were analyzed in 90 recombinant progeny, statistically significant, locus-specific deviations from expected Mendelian transmission ratios were observed for a sizeable fraction of all markers in both reciprocal crosses: some markers were preferentially transmitted by the J strain, while others were preferentially transmitted by the I strain. It is speculated that these transmission distortions may be related to the regions of inter-isolate genetic incompatibility, and may complicate the use of J x I crosses to establish a RFLP-based linkage map for the species.