Population genetic diversity and climate/habitat stability
We found higher nuclear and mitochondrial diversity in the Northern,
Central and Eastern population, than in Southern populations (Figure 4;
Table S8). Linear regression analyses revealed that nuclear genetic
diversity was significantly correlated with latitude for nDNA but not
with longitude (Table 3). Conversely, we found that mtDNA genetic
diversity was not correlated with latitude nor with longitude. Likewise,
analyses showed that all nuclear genetic diversity estimates
(H E, π) were positively correlated with the three
environmental stability estimates (StabilityENV,
StabilityHOST and StabilityENV-HOST)
from the LGM (~21 Kya) to the present, but mitochondrial
genetic diversity estimators (HD, π) were not correlated
(Table 3; Figure 4).
Discussion
Nuclear SNPs and mtDNA data revealed significant population structure
across the range of C. cactorum . Landscape genomic analyses
provided support for the hypothesis that a combination of environmental
conditions and habitat suitability, influenced by shifts in host species
distributions during the Last Glacial Maximum (LGM), were the main
forces shaping population genetic structure of C. cactorum across
its native Argentine range. Shifts in the distribution of Opuntiahost species, mediated by climatic changes during the Quaternary, are
suggested to have had a direct influence on the distribution of C.
cactorum , creating fragmented ranges that led to reduced gene flow
among populations, promoting genetic differentiation. Results also
supported the habitat stability hypothesis, whereby regions within which
suitable environments have remained stable since the LGM to the present
harbored more genetic diversity than regions of lower habitat stability.
Relatively higher habitat stability within the northern region of the
distribution of C. cactorum created refuge conditions promoting
the maintenance of higher levels of genetic diversity over time,
compared to other areas. The hypothesis that the geographically
widespread cultivation of O. ficus-indica facilitated
contemporary gene flow among otherwise geographically distant
populations was rejected, based on population genomic analyses revealing
limited genetic admixture and a hierarchical pattern of population
differentiation concordant with geography.