Figure 5.

The JU1825 cytoplasm is heteroplasmic for JU1825-like and NIC59-like alleles. (A) The viability of independent (J); N/J female × NIC59 male backcross lineages was followed until the F5 generation. Surprisingly, in some lineages, multiple generations of backcrossing resulted in increased viability (similar to that seen in intrastrain crosses). Number of independent backcross lineages assayed per generation: F2 = 15, F3 = 15, F4 = 14, F5 = 14. (B) The viability of independent (J); N/J female × JU1825 male backcross lineages was also followed until the F5 generation. Interestingly, multiple generations of backcrossing resulted in some lineages with significantly reduced viability, similar to that seen in (N); N/J F1 female × JU1825 male crosses. Number of independent backcross lineages assayed per generation, F2–F5 = 14. (C) Embryonic lethality of the same (J); N/J female × JU1825 male backcross lineages from (B) (with same color-coding). Upon additional generations of backcrossing, some (J); N/J female × JU1825 male lineages exhibit ∼50% embryonic lethality, similar to (N); N/J F1 female × JU1825 male crosses. These results are consistent with the hypothesis that the JU1825 cytoplasm is heteroplasmic and contains JU1825-like and NIC59-like alleles.