The influence of climatic niche preferences on the population genetic structure of a mistletoe species complex

Abstract

The prevalent view on genetic structuring in parasitic plants is that host-race formation is caused by varying degrees of host specificity. However, the relative importance of ecological niche divergence and host specificity to population differentiation remains poorly understood. We evaluated the factors associated with population differentiation in mistletoes of the Psittacanthus schiedeanus complex (Loranthaceae) in Mexico. We used genetic data from chloroplast sequences and nuclear microsatellites to study population genetic structure and tested its association with host preferences and climatic niche variables. Pairwise genetic differentiation was associated with environmental and host preferences, independent of geography. However, environmental predictors appeared to be more important than host preferences to explain genetic structure, supporting the hypothesis that the occurrence of the parasite is largely determined by its own climatic niche and, to a lesser degree, by host specificity. Genetic structure is significant within this mistletoe species complex, but the processes associated with this structure appear to be more complex than previously thought. Although host specificity was not supported as the major determinant of population differentiation, we consider this to be part of a more comprehensive ecological model of mistletoe host-race formation that incorporates the effects of climatic niche evolution.

Hybridization and differential introgression associated with environmental shifts in a mistletoe species complex

“…Accordingly, the most accepted explanation of mistletoe diversification is that of ‘host-race formation’ 12 , 34 – 36 , where genetic differentiation, and eventually host-race formation, is acquired through isolation-by-distance or by ecological adaptation following the ‘invasion’ of a different host species (‘host-switching’ hypothesis 12 ). In addition, the geographic structuring of genetic variation in some mistletoe species has been explained as the result of past climate changes 31 , 32 , 37 , 38 , landscape fragmentation 39 , emergence of biogeographic barriers 30 – 32 , 38 , and by the parasites’ own climatic niche preferences 40 , 41 .…”

“…schiedeanus ( c . 2.5–1.8 Ma 32 , 38 ) can be found infecting different host tree species under distinct environmental conditions along their wide geographical distributions 29 , 41 , 58 – 60 . Kuijt 21 reported that these species are distributed, more or less sympatrically, from Mexico to Panama, but recent molecular data suggest that these are allopatric and restricted to Mexico 41 .…”

“…schiedeanus parasitizes cloud forest trees, mainly Liquidambar styraciflua (Altingiaceae) in eastern Mexico 32 , 38 , 41 . Patterns of genetic structure in chloroplast DNA (cpDNA) sequences and nuclear microsatellites showed two main lineages within the species complex 32 , 38 , 41 , with further genetic structuring of populations and evidence of genetic admixture in particular regions 41 . However, the presence of highly admixed individuals within particular populations has not been explored or discussed further 41 .…”

“…Patterns of genetic structure in chloroplast DNA (cpDNA) sequences and nuclear microsatellites showed two main lineages within the species complex 32 , 38 , 41 , with further genetic structuring of populations and evidence of genetic admixture in particular regions 41 . However, the presence of highly admixed individuals within particular populations has not been explored or discussed further 41 . Despite clear geographic and ecological differences between the ‘ calyculatus ’ and ‘ schiedeanus ’ genetic clusters, molecular evidence suggests population admixture within the Tehuacán-Cuicatlán Valley and the Central Valleys of Oaxaca, and the Central Depression of Chiapas 32 , 38 , 41 .…”

“…However, the presence of highly admixed individuals within particular populations has not been explored or discussed further 41 . Despite clear geographic and ecological differences between the ‘ calyculatus ’ and ‘ schiedeanus ’ genetic clusters, molecular evidence suggests population admixture within the Tehuacán-Cuicatlán Valley and the Central Valleys of Oaxaca, and the Central Depression of Chiapas 32 , 38 , 41 . Overall, these regions have a drier climate and more xeric vegetation types than those occurring elsewhere at higher elevations.…”