Footprints of historical admixture
Despite the aforementioned distributional shifts of host species resulting from Quaternary climate dynamics, population genetic analyses revealed evidence for gene flow among Chacoan (Central lineage) and North Pampean (East lineage) populations of C. cactorum . In particular, extensive nuclear admixture and sharing of mtDNA haplotypes occurred across those populations located along the contact area between northern Pampa and Chaco (Figure 2). This area coincides with a transition between Chaco and Pampa environments, referred to as the Espinal (Bucher, 1982). Historical secondary contact among previously isolated Central and East populations, likely prompted by post glacialOpuntia host range expansion, provides a plausible explanation for the observed pattern of genetic admixture. Evidence for admixture was also detected between populations of East and South lineages, both distributed within the Pampean and Monte regions. This area has been reported as a landscape corridor facilitating connectivity between previously separated populations of the grasshopper species D. vittatus inhabiting grassland and savanna biomes (Rosetti et al., 2022). Our results would further emphasize the extensive habitat connectivity between these regions during the Quaternary. Overall, our results argue against the idea that the human-driven introduction and intensive cultivation of O. ficus-indica enable rapid expansion of C. cactorum , promoting contemporary gene flow among geographically distant populations.
Landscape genomic analyses
In agreement with inferences of divergence times among three major lineages as inferred by coalescent-based demographic modeling, the landscape genomic analyses revealed that the spatial pattern of population genetic differentiation was best explained by a Quaternary landscape scenario representing the distribution of climatically suitable habitats and predicted ranges of host species during the LGM (Figure 1 and 3). The influence of historical landscape composition on the contemporary genomic variation pattern was illustrated by the fact that weakly differentiated populations were predicted to have had high habitat connectivity in LGM projections. This is the case of JUJ (northwestern) and FOG (northeastern) populations (FST=0.088; Table S6), currently separated by 740 km but subjected to high levels of past habitat connectivity according to suitability maps. In contrast, a discontinuous habitat was expected to promote isolation and, thus, genetic differentiation even between geographically proximate populations. This would be particularly so forC. cactorum , given its presumably limited dispersal ability because of their short-lived adult stage (Pettey, 1948; Zimmerman, Moran & Hoffmann, 2000), and illustrated by the southern populations LPS (southwestern) and BAP (southeastern) which exhibit one of the highest FST values (FST=0.389), despite the relatively limited geographic distance separating them of approximately 290 km. Environmental projections during LGM suggest that dispersal between LPS and BAP was likely limited due to low environmental suitability, thus limiting gene flow during glacial periods. Thus, our results would indicate that environmental tolerance together with limited dispersal could have interacted with landscape features to generate population genetic structure (Broquet & Petit, 2009; Sherpa et al., 2020).
Despite signatures for shifts in host species distributions, which may have influenced contemporary population genetic structure in C. cactorum, host species use appeared to be less of a consequential influence. This is at odds with observations in other insect herbivores (Forbes et al., 2017; Funk et al., 2006; Poveda-Martínez et al., 2020), but in line with field and laboratory host range studies (Varone et al 2014). In areas where there is more than one Opuntia species, host use by moths was proportional to host species abundance. Additionally, multiple choice experiments revealed that female C. cactorum do not exhibit oviposition preference for Opuntiaspecies. Together, these results suggest that host species is not an important selective agent for C. cactorum , consistent with evidence from other generalist herbivores (Vidal & Murphy, 2018; Vidal et al., 2019).