Decoupled mitochondrial and chloroplast DNA population structure reveals Holocene collapse and population isolation in a threatened Mexican‐endemic conifer

Abstract

Chihuahua spruce (Picea chihuahuana Martínez) is a montane subtropical conifer endemic to the Sierra Madre Occidental in northwestern México. Range-wide variation was investigated using maternally inherited mitochondrial (mtDNA) and paternally inherited chloroplast (cpDNA) DNA markers. Among the 16 mtDNA regions analysed, only two mitotypes were detected, while the study of six cpDNA microsatellite markers revealed eight different chlorotypes. The average cpDNA diversity (H = 0.415) was low but much higher than that for mtDNA (H = 0). The distribution of mitotypes revealed two clear nonoverlapping areas (G(ST) = N(ST) = 1), one including northern populations and the second one including the southern and central stands, suggesting that these two regions may represent different ancestral populations. The cpDNA markers showed lower population differentiation (G(ST) = 0.362; R(ST) = 0.230), implying that the two ancestral populations continued to exchange pollen after their initial geographic separation. A lack of a phylogeographic structure was revealed by different spatial analyses of cpDNA (G(ST) > R(ST); and samova), and reduced cpDNA gene flow was noted among populations (Nm = 0.873). Some stands deviated significantly from the mutation-drift equilibrium, suggesting recent bottlenecks. Altogether, these various trends are consistent with the hypothesis of a population collapse during the Holocene warming and suggest that most of the modern P. chihuahuana populations are now effectively isolated with their genetic diversity essentially modelled by genetic drift. The conservation efforts should focus on most southern populations and on the northern and central stands exhibiting high levels of genetic diversity. Additional mtDNA sequence analysis confirmed that P. martinezii (Patterson) is not conspecific with P. chihuahuana, and thus deserves separate conservation efforts.

Spatial genetic structure in the very rare and species-rich Picea chihuahuana tree community (Mexico)

“…Such features should prompt genetic drift resulting in high genetic divergence (HAMPE and PETIT, 2005), as previously shown for P. chihuahuana with both allozymes and chloroplast markers (LEDIG et -Perez et. al.·Silvae Genetica (2014) 63-4, 149-159 al., 1997; JARAMILLO-CORREA et al, 2006). For P. chihuahuana and P. menziesii at the large scale, our results provided strong support for the theory of isolation by distance that predicts the expected pattern of SGS at drift-dispersal equilibrium (VEKEMANS and HARDY, 2004).…”

“…The P. chihuahuana tree community has been the subject of several studies concerning its ecology and population genetics. The overall aim of most of these studies has been to obtain data to help design preservation and conservation strategies (NARVÁEZ, 1984;LEDIG et al, 1997;LEDIG et al, 2000;JARAMILLO-CORREA et al, 2006;WEHENKEL et al, 2012;WEHENKEL and SÁENZ-ROMERO, 2012;QUIÑONES-PÉREZ et al, 2012;MENDOZA-MAYA et al, 2015). However, analysis of the fine-scale SGS in this special forest tree community has not yet been conducted, which might help enrich the abovementioned conservation programs.…”

Vegetación de la Sierra Madre Occidental, México: una síntesis

“…Esta especie es de distribución geográfica muy restringida, relictual, endémica a la Sierra Madre Occidental (Jaramillo-Correa et al, 2006;García Arévalo, 2008). De distribución previa más amplia cuando las condiciones ambientales de mayor humedad y temperaturas relativamente bajas prevalecían en buena parte de México, hace unos 9000 años, casi desapareció del área (Gordon 1968;Wehenkel y Saenz-Romero, 2012).…”

Modeling the Potential Distribution of Picea chihuahuana Martínez, an Endangered Species at the Sierra Madre Occidental, Mexico

“…Our results can be used for planning biological corridors, reseeding and transplanting at southern populations, which are small, isolated and with less genetic variability [36,37]. Meanwhile, the identification of potential habitat may help P. chihuahuana conservation by protecting it from any threat (i.e., fragmentation, logging) at northern populations.…”