Fitness recovery
Both control parental lines went extinct after two generations of
experimental evolution. In contrast, all hybrid lines survived the nine
months of experimental evolution, with the exception of SC3, which went
extinct in the 6th month. Most of the surviving lines showed an increase
in population size (6/9 of the SC lines, and 6/7 of the SD lines), with
census size showing an increase between 1.1-fold in SD4 to 9-fold in SD6
(Fig. 2A).
Regarding fecundity (Fig. 2B), F2 hybrids had lower fitness than the
parental with the respective mitochondrial lineage, confirming F2 hybrid
breakdown in this life history trait. Hybrid breakdown was more extreme
in the comparison with the SC mitochondria (P-value= 0.005), relative to
that with the SD mitochondria (P-value= 0.017), consistent with stronger
breakdown in SC mitochondrial background. Of the hybrid lines that
survived experimental evolution, 7 SC and 6 SD lines had sufficient
virgin females to assess fecundity. Mean fecundity ±1SE of all hybrid
lines was within the range observed in the respective F2 hybrids (mean
plus ±1SE), consistent with no or little recovery from initial hybrid
breakdown.
Regarding survivorship (Fig. 2C), both reciprocal hybrids had lower
fitness than parental populations with the respective mitochondrial
lineage, yet non-significant (P-values> 0.125), confirming
breakdown in this trait. The ±1SE intervals overlapped in SD but not in
the SC comparison, again suggesting that F2 hybrid breakdown is stronger
in the SC mitochondrial background. A total of 8 SC and 6 SD lines had
sufficient gravid females to assess survivorship of nauplii. Of these, 4
SC lines and 4 SD lines had survivorship (mean ±1SE) at or above the
fitness of the respective parental, suggesting recovery from the initial
hybrid breakdown. These differences were significant for the SC4, SC7,
SC8, SD3 and SD7 lines (respective corrected P-values= 0.001, 0.009,
0.017, 0.043, 0.043; Fig. 2C).
Based on fewer replicates measured during the experiment, we observed a
large temporal variability on survivorship (Fig. 2D, Fig. S1). The
increase of survivorship was more noticeable among SC lines, where F2
breakdown was stronger, and lines stayed for a few generations at the
fitness level of their parental population.
Based on these results, for the lines evolving under the SC
mitochondria, we chose to sequence SC4, SC8 and SC10 because, from lines
with the highest increase of population size, these were the ones with
highest survivorship at the end of the experiment. For the lines
evolving under the SD mitochondria, we chose to sequence SD3, SD4, and
SD6 because they showed some increase in survivorship, without a
noticeable decrease in fecundity (contrary to SD7). We note that,
although the choice of lines is necessarily subjective, given that this
experimental design did not employ direct selection on a specific
phenotype, surviving lines showing population growth would be good
representatives of recovery from initial F2 breakdown.