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1. Contrasting effects of host tree isolation on population connectedness in two tropical epiphytic bromeliads

   https://doi.org/10.1002/ajb2.1391  

   Amici, Autumn A. ; Nadkarni, Nalini M. ; DiBlasi, Emily ; Seger, Jon ( December 2019 , American Journal of Botany)

   Conversion of primary forests to pastures is a major cause of habitat fragmentation in the tropics. Fragmentation is expected to impede gene flow for many plant species that are restricted to remaining forest fragments. Epiphytes may be especially vulnerable to this effect of forest fragmentation because they depend on host trees. However, trees that remain in pastures may enhance connectivity across the landscape for epiphyte species that can thrive on such trees. To investigate this possibility, we studied the genetic structures of two such species on isolated pasture trees and surrounding forest, in relation to their local abundances in different habitat types and aspects of their reproductive biology including pollen and seed dispersal agents, and looked for evidence of increased or diminished gene flow.

   We used microsatellite markers to assess geographic patterns of genetic diversity and differentiation in two epiphytic bromeliads,Catopsis nitidaandWerauhia tonduziana,in the Monteverde region of Costa Rica.

   About 85% of theF_STvalue forCatopsis nitidawas found among pastures within regions, while forWeruahia tonduziana,about 80% of theF_STvalue was contributed by differences between regions, indicating much more gene flow within regions, relative toC. nitida.

   Although there was substantial genetic differentiation among epiphyte populations, those on isolated pasture trees were not substantially less diverse than those in adjacent forests, suggesting that pasture trees may serve as “stepping stones” that help these species maintain their genetic connectedness and diversity at larger geographic scales.

    

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2. Conservation genetics and breeding system of Penstemon debilis (Plantaginaceae), a rare beardtongue endemic to oil shale talus in western Colorado, USA

   https://doi.org/10.1111/jse.12100  

   WOLFE, Andrea D. ; MCMULLEN‐SIBUL, Amy ; TEPEDINO, V. J. ; KUBATKO, Laura ; NECAMP, Timothy ; FASSNACHT, Susan ( May 2014 , Journal of Systematics and Evolution)

   Rare species of plants are especially vulnerable to extinction when populations are few, have small numbers of individuals, and are fragmented. Such conditions lead to a reduction in gene flow and genetic diversity, and encourage inbreeding depression. We conducted a study of the reproductive biology and population genetics ofPenstemon debilis(Plantaginaceae), a Federally Threatened species endemic to a small region of oil shale extraction in western Colorado, USA. Most of the habitat area is privately owned and undergoing natural gas extraction activities.Penstemon debilisreproduces both vegetatively and as an outcrosser that requires a pollen vector. Moderate levels of inbreeding, but no inbreeding depression, were found within populations ofP. debilis. Genetic divergence among the extant populations surveyed was moderate (F_STvalues = 0.069–0.231;Nm = 0.831–3.385) with levels of genetic diversity within populations relatively low compared to congeners with similar modes of pollination and reproductive biology. STRUCTURE analysis revealed three population clusters with some admixture among all extant populations. Genetic diversity within and amongP. debilispopulations is similar to genetic diversity found for other rare and endemic outcrossing plant species. Our results are consistent with a pattern of recent population fragmentation or low levels of pollen‐mediated gene flow among populations in close proximity to one another. Conservation ofP. debiliswill require cooperative management strategies between private landowners, government agencies, and concerned NGOs to preserve habitat for this rare species.

    

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3. Heterosis is common and inbreeding depression absent in natural populations of Arabidopsis thaliana

   https://doi.org/10.1111/jeb.13441  

   Oakley, Christopher G. ; Lundemo, Sverre ; Ågren, Jon ; Schemske, Douglas W. ( April 2019 , Journal of Evolutionary Biology)

   The importance of genetic drift in shaping patterns of adaptive genetic variation in nature is poorly known. Genetic drift should drive partially recessive deleterious mutations to high frequency, and inter‐population crosses may therefore exhibit heterosis (increased fitness relative to intra‐population crosses). Low genetic diversity and greater genetic distance between populations should increase the magnitude of heterosis. Moreover, drift and selection should remove strongly deleterious recessive alleles from individual populations, resulting in reduced inbreeding depression. To estimate heterosis, we crossed 90 independent line pairs ofArabidopsis thalianafrom 15 pairs of natural populations sampled across Fennoscandia and crossed an additional 41 line pairs from a subset of four of these populations to estimate inbreeding depression. We measured lifetime fitness of crosses relative to parents in a large outdoor common garden (8,448 plants in total) in central Sweden. To examine the effects of genetic diversity and genetic distance on heterosis, we genotyped parental lines for 869 SNPs. Overall, genetic variation within populations was low (median expected heterozygosity = 0.02), and genetic differentiation was high (medianF_ST = 0.82). Crosses between 10 of 15 population pairs exhibited significant heterosis, with magnitudes of heterosis as high as 117%. We found no significant inbreeding depression, suggesting that the observed heterosis is due to fixation of mildly deleterious alleles within populations. Widespread and substantial heterosis indicates an important role for drift in shaping genetic variation, but there was no significant relationship between fitness of crosses relative to parents and genetic diversity or genetic distance between populations.

    

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4. Testing main Amazonian rivers as barriers across time and space within widespread taxa

   https://doi.org/10.1111/jbi.13676  

   Pirani, Renata M. ; Werneck, Fernanda P. ; Thomaz, Andréa T. ; Kenney, Mariah L. ; Sturaro, Marcelo J. ; Ávila‐Pires, Teresa C. S. ; Peloso, Pedro L. V. ; Rodrigues, Miguel T. ; Knowles, L. Lacey ( September 2019 , Journal of Biogeography)

   Present Amazonian diversity patterns can result from many different mechanisms and, consequently, the factors contributing to divergence across regions and/or taxa may differ. Nevertheless, the river‐barrier hypothesis is still widely invoked as a causal process in divergence of Amazonian species. Here we use model‐based phylogeographic analyses to test the extent to which major Amazonian rivers act similarly as barriers across time and space in two broadly distributed Amazonian taxa.

   Amazon rain forest.

   The lizardGonatodes humeralis(Sphaerodactylidae) and the tree frogDendropsophus leucophyllatus(Hylidae).

   We obtained RADseq data for samples distributed across main river barriers, representing main Areas of Endemism previously proposed for the region. We conduct model‐based phylogeographic and genetic differentiation analyses across each population pair.

   Measures of genetic differentiation (based onF_STcalculated from genomic data) show that all rivers are associated with significant genetic differentiation. Parameters estimated under investigated divergence models showed that divergence times for populations separated by each of the 11 bordering rivers were all fairly recent. The degree of differentiation consistently varied between taxa and among rivers, which is not an artifact of any corresponding difference in the genetic diversities of the respective taxa, or to amounts of migration based on analyses of the site‐frequency spectrum.

   Taken together, our results support a dispersal (rather than vicariance) history, without strong evidence of congruence between these species and rivers. However, once a species crossed a river, populations separated by each and every river have remained isolated—in this sense, rivers act similarly as barriers to any further gene flow. This result suggests differing degrees of persistence and gives rise to the seeming contradiction that the divergence process indeed varies across time, space and species, even though major Amazonian rivers have acted as secondary barriers to gene flow in the focal taxa.

    

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5. Conservation genetic assessment of the paleback darter, Etheostoma pallididorsum , a narrowly distributed endemic in the Ouachita Highlands, Arkansas, USA

   https://doi.org/10.1002/aqc.3510  

   McCall, Brittany L. ; Fluker, Brook L. ( March 2021 , Aquatic Conservation: Marine and Freshwater Ecosystems)

   The paleback darter,Etheostoma pallididorsum, is considered imperilled and has recently been petitioned for listing under the Endangered Species Act. Previous allozyme‐based studies found evidence of a small effective population size, warranting conservation concern. The objective of this study was to assess the population dynamics and the phylogeographical history of the paleback darter, using a multilocus microsatellite approach and mitochondrial DNA.

   The predictions of this study were that: paleback darter populations will exhibit low genetic diversity and minimal gene flow; population structure will correspond to the river systems from which the samples are derived; reservoir dams impounding the reaches between the Caddo and Ouachita rivers would serve as effective barriers to gene flow; and the Caddo and Ouachita rivers are reciprocally monophyletic.

   Microsatellite DNA loci revealed significant structure among sampled localities (globalF_st= 0.17,P< 0.001), with evidence of two distinct populations representing the Caddo and Ouachita rivers. However, Bayesian phylogeographical analyses resulted in three distinct clades: Caddo River, Ouachita River, and Mazarn Creek. Divergence from the most recent ancestor shared among the river drainages was estimated at 60 Kya. Population genetic diversity was relatively low (H_e= 0.65; mean alleles per locus,A= 6.26), but was comparable with the population genetic diversity found in the close relatives slackwater darter,Etheostoma boschungi(H_e= 0.65;A= 6.74), and Tuscumbia darter,Etheostoma tuscumbia(H_e= 0.57;A= 5.53).

   These results have conservation implications for paleback darter populations and can be informative for other headwater specialist species. Like other headwater species with population structuring and relatively low genetic diversity, the persistence of paleback darter populations is likely to be tied to the persistence and connectivity of local breeding and non‐breeding habitat. These results do not raise conservation concern for a population decline; however, the restricted distribution and endemic status of the species still renders paleback darter populations vulnerable to extirpation or extinction.

    

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