We used mitochondrial DNA (mtDNA) sequence data and allele frequencies at eight microsatellite loci to examine the population genetic structure, estimate the divergence times of distinct lineages, and infer patterns associated with host colonization in populations of the bark beetle Dendroctonus approximatus in Mexico. Two haplotype groups were identified using mtDNA sequences in 71 individuals from 15 populations. The first group was distributed in the Sierra Madre Occidental (SMOc, Western Mexico), with some populations in the Faja Volcánica Transmexicana (Central Mexico), and the second was found in the Sierra Madre Oriental (SMOr, Eastern Mexico), with populations in the Sierra Madre del Sur (Southern Mexico). The estimated split between groups occurred in the late Pleistocene, around 0.195 Mya. Microsatellite allele frequencies revealed high genetic differentiation between pairwise populations, and genetic differentiation values indicated a genetic structure of isolation by distance. Both mtDNA sequence data and microsatellite allele frequencies indicated that D. approximatus had two independent colonization routes in Mexico, one through the SMOc and another along the SMOr. The widespread geographic distribution of D. approximatus in Mexico follows a model of population range expansion of two haplotype groups in which gene flow is restricted by the geographic separation between hosts imposed by physical barriers between populations.

The mechanisms generating the geographical distributions of genetic diversity are a central theme in evolutionary biology. The amount of genetic diversity and its distribution are controlled by several factors, including dispersal abilities, physical barriers, and environmental and climatic changes. We investigated the patterns of genetic diversity and differentiation among populations of the widespread species Brosimum alicastrum in Mexico. Methods: Using nuclear DNA microsatellite data, we tested whether the genetic structure of B. alicastrum was associated with the roles of the Trans-Mexican Volcanic Belt and the Isthmus of Tehuantepec as geographical barriers to gene flow and to infer the role of past events in the genetic diversity patterns. We further used a maximumlikelihood population-effects mixed model (MLPE) to identify the main factor affecting population differentiation in B. alicastrum. Results: Our results suggested that Mexican B. alicastrum is well differentiated into three main lineages. Patterns of the genetic structure at a finer scale did not fully correspond to the current geographical barriers to gene flow. According to the MLPE mixed model, isolation by distance is the best model for explaining the genetic differentiation of B. alicastrum in Mexico. Conclusions: We propose that the differentiation patterns might reflect (1) an ancient differentiation that occurred in Central and South America, (2) the effects of past climatic changes, and (3) the functions of some physical barriers to gene flow. This study provides insights into the possible mechanisms underlying the geographic genetic variation of B. alicastrum along a moisture gradient in tropical lowland forests.

Background: Genetic diversity is needed to preserve the capability of a species to survive to environmental changes. Due to the presence of small isolated populations, relict species such as Magnolia are at an elevated extinction risk. In recent years, many new species of Magnolia have been described in Mexico, each one classified by its category of risk. To achieve conservation, knowledge of their basic level of biological diversity is essential to design adequate conservation plans and avoid the negative consequences of genetic loss. Here, we implemented nuclear microsatellite markers to assess 13 populations of three new species of Magnolia that were all previously considered to be Magnolia pacifica. We aimed to evaluate the genetic agreement with the distinction of these three different morphological species (e.g., their species integrity) and to determine their levels of genetic diversity and their geographic distribution to propose conservation strategies. Results: We found high levels of genetic diversity compared to other Magnolia species with no sign of inbreeding. We found a small effective population size and a prevalence of bottlenecks in some populations. The patterns of genetic subdivision did not support the current morphological distinction of three different species. Instead, we suggest that the genetic structure pattern is the result of historical connectivity and the continuous natural fragmentation of the forest. Thus, an isolation by distance pattern may have had an important role in shaping allele frequencies, producing local genetic differences. Conclusions: We argue that a major threat underlies the actual trends of habitat loss, which can directly impact the loss of genetic diversity in the current adult individuals and consequently, increase the risk of extinction in further generations. For conservation purposes, we suggest combining in situ and ex situ conservation of populations with the maintenance of connectivity among the local populations.