Tall Pinus luzmariae trees with genes from P. herrerae

Stochastic processes and changes in evolutionary rate are associated with diversification in a lineage of tropical hard pines (Pinus)

The study of the patterns of polymorphism and molecular evolution among closely related species is key to understanding the evolutionary forces involved in the diversification of lineages. This point is a big challenge in species with slow evolutionary rates, long life cycles, and ancient, shared polymorphisms such as conifers. Under the premise of divergence in a stepwise migration process, we expect clinal geographical patterns of purifying selection efficiency, and genetic structure related to latitude or longitude. If migration is accompanied by changes in the environment, we could further expect a role of positive selection in driving species divergence. Here, we infer patterns of polymorphism, efficiency of purifying selection, and molecular evolution using a dataset of 161 nuclear genes (∼71 Kb) in a lineage of hard pines from North America, the Caribbean, Mexico, and Central America presumed to have migrated from North America toward lower latitudes with tropical conditions. Under the premise of differences in selective pressures, we also look for possible signals of positive selection. To test our hypothesis, first we estimated different indices to infer patterns of polymorphism and efficiency of purifying selection (Ka, Ks, Ka/Ks, dN, dS, dN/dS, and dxy) and compared these metrics across five clades. Also, we investigated possible clinal patterns in these indices and morphological traits (needle length and cone length). Then we inferred genetic structure and environmental differences among species to test for possible signals of positive selection using phylogenetic methods in specific clades. We found differences among clades using Ka, Ks, and Ka/Ks with a relaxation of purifying selection, especially in the Elliotti and Patula clades. We also found environmental differences related to geographic distance, and among clades suggesting differences in selective pressures. The indices Ks, dxy, and needle length had relationships with geography but not ovulate cone length. Finally, we found that most analyzed genes are under purifying selection, but there was an exception of faster evolutionary rate in some pine species, suggesting the possible action of positive selection in divergence. Our study indicated that stochastic processes have played a key role in the diversification of the group, with a possible input of positive selection in pines from Mexico and Central America.

Betula mcallisteri sp. nov. (sect. Acuminatae, Betulaceae), a new diploid species overlooked in the wild and in cultivation, and its relation to the widespread B. luminifera

Taxa are traditionally identified using morphological proxies for groups of evolutionarily isolated populations. These proxies are common characters deemed by taxonomists as significant. However, there is no general rule on which character or sets of characters are appropriate to circumscribe taxa, leading to discussions and uncertainty. Birch species are notoriously hard to identify due to strong morphological variability and factors such as hybridization and the existence of several ploidy levels. Here, we present evidence for an evolutionarily isolated line of birches from China that are not distinguishable by traditionally assumed taxon recognition proxies, such as fruit or leaf characters. We have discovered that some wild material in China and some cultivated in the Royal Botanic Gardens Edinburgh, formerly recognized as Betula luminifera, differ from other individuals by having a peeling bark and a lack of cambial fragrance. We use restriction site-associated DNA sequencing and flow cytometry to study the evolutionary status of the unidentified Betula samples to assess the extent of hybridization between the unidentified Betula samples and typical B. luminifera in natural populations. Molecular analyses show the unidentified Betula samples as a distinct lineage and reveal very little genetic admixture between the unidentified samples and B. luminifera. This may also be facilitated by the finding that B. luminifera is tetraploid, while the unidentified samples turned out to be diploid. We therefore conclude that the samples represent a yet unrecognized species, which is here described as Betula mcallisteri.

Assisted migration and the rare endemic plant species: the case of two endangered Mexican spruces

Background

In the projected climate change scenarios, assisted migration might play an important role in the ex situ conservation of the threatened plant species, by translocate them to similar suitable habitats outside their native distributions. However, it is unclear if such habitats will be available for the Rare Endemic Plant Species (REPS), because of their very restricted habitats. The aims of this study were to perform a population size assessment for the REPS Picea martinezii Patterson and Picea mexicana Martínez, and to evaluate the potential species distributions and their possibilities for assisted migration inside México and worldwide.

Methods

We performed demographic censuses, field surveys in search for new stands, and developed distribution models for Last Glacial Maximum (22,000 years ago), Middle Holocene (6,000 years ago), current (1961-1990) and future (2050 and 2070) periods, for the whole Mexican territory (considering climatic, soil, geologic and topographic variables) and for all global land areas (based only on climate).

Results

Our censuses showed populations of 89,266 and 39,059 individuals for P. martinezii and P. mexicana, respectively, including known populations and new stands. Projections for México indicated somewhat larger suitable areas in the past, now restricted to the known populations and new stands, where they will disappear by 2050 in a pessimistic climatic scenario, and scarce marginal areas (p = 0.5-0.79) remaining only for P. martinezii by 2070. Worldwide projections (based only on climate variables) revealed few marginal areas in 2050 only in México for P. martinezii, and several large areas (p ≥ 0.5) for P. mexicana around the world (all outside México), especially on the Himalayas in India and the Chungyang mountains in Taiwan with highly suitable (p ≥ 0.8) climate habitats in current and future (2050) conditions. However, those suitable areas are currently inhabited by other endemic spruces: Picea smithiana (Wall.) Boiss and Picea morrisonicola Hayata, respectively.

Conclusions

Assisted migration would only be an option for P. martinezii on scarce marginal sites in México, and the possibilities for P. mexicana would be continental and transcontinental translocations. This rises two possible issues for future ex situ conservation programs: the first is related to whether or not consider assisted migration to marginal sites which do not cover the main habitat requirements for the species; the second is related to which species (the local or the foreign) should be prioritized for conservation when suitable habitat is found elsewhere but is inhabited by other endemic species. This highlights the necessity to discuss new policies, guidelines and mechanisms of international cooperation to deal with the expected high species extinction rates, linked to projected climate change.

Graft survival of Pinus engelmannii Carr. in relation to two grafting techniques with dormant and sprouting buds

Developing methods for successfully grafting forest species will be helpful for establishing asexual seed orchards and increasing the success of forest genetic improvement programs in Mexico. In this study we investigated the effects of two grafting techniques (side veneer and top cleft) and two phenological stages of the scion buds (end of latency and beginning of sprouting), in combination with other seven grafting variables, on the sprouting and survival of 120 intraspecific grafts of Pinus engelmannii Carr. The scions used for grafting were taken from a 5.5-year-old commercial forest plantation. The first grafting was performed on January 18 (buds at the end of dormancy) and the second on February 21 (buds at the beginning of sprouting). The data were examined by analysis of variance and a test of means and were fitted to two survival models (the Weibull's accelerated failure time and the Cox's proportional hazards model) and the respective hazard ratios were calculated. Survival was higher in the top cleft grafts made with buds at the end of latency, with 80% sprouting and an estimated average survival time of between 164 and 457 days after the end of the 6-month evaluation period. Four variables (grafting technique, phenological stage of the scion buds, scion diameter and rootstock height) significantly affected the risk of graft death in both survival models. Use of top cleft grafts with buds at the end of the latency stage, combined with scion diameters smaller than 11.4 mm and rootstock heights greater than 58.5 cm, was associated with a lower risk of death.