Flexible drought deciduousness in a neotropical understory herb.

PREMISE: Adaptive divergence across environmental gradients is a key driver of speciation. Precipitation seasonality gradients are common in the tropics, yet drought adaptation is nearly unexplored in neotropical understory herbs. Here, we examined two recently diverged neotropical spiral gingers, one adapted to seasonal drought and one reliant on perennial water, to uncover the basis of drought adaptation. METHODS: We combined ecophysiological trait measurements in the field and greenhouse with experimental and observational assessments of real-time drought response to determine how Costus villosissimus (Costaceae) differs from C. allenii to achieve drought adaptation. RESULTS: We found that drought-adapted C. villosissimus has several characteristics indicating flexible dehydration avoidance via semi-drought-deciduousness and a fast economic strategy. Although the two species do not differ in water-use efficiency, C. villosissimus has a more rapid growth rate, lower leaf mass per area, lower stem density, higher leaf nitrogen, and a strong trend of greater light-saturated photosynthetic rates. These fast economic strategy traits align with both field-based observations and experimental dry-down results. During drought, C. villosissimus displays facultative drought-deciduousness, losing lower leaves during the dry season and rapidly growing new leaves in the wet season. CONCLUSIONS: We revealed a drought adaptation strategy that has not, to our knowledge, previously been documented in tropical herbs. This divergent drought adaptation evolved recently and is an important component of reproductive isolation between C. villosissimus and C. allenii, indicating that adaptive shifts to survive seasonal drought may be an underappreciated axis of neotropical understory plant diversification.

PREMISA DEL ESTUDIO: La divergencia adaptativa a lo largo de gradientes ambientales es un factor clave de la especiación. Los gradientes de estacionalidad de la precipitación son comunes en los trópicos, sin embargo, la adaptación a la sequía es casi inexplorada en las hierbas neotropicales del sotobosque. Examinamos dos especies de caña agria neotropicales que divergieron recientemente, uno adaptado a la sequía estacional y otro que depende del agua perenne, para descubrir la base de la adaptación a la sequía. MÉTODOS: Combinamos mediciones ecofisiológicas en el campo y el invernadero con una evaluación experimental y observacional de la respuesta a la sequía en tiempo real para determinar cómo Costus villosissimus (Costaceae) difiere de C. allenii para lograr la adaptación a la sequía. RESULTADOS CLAVE: Encontramos que C. villosissimus, que está adaptado a la sequía, tiene varias características que indican que evita la deshidratación a través de la caducididad y una estrategia de vida rápida. Aunque las dos especies no difieren en la eficiencia del uso del agua, C. villosissimus tiene una tasa de crecimiento más rápida, menor masa foliar por área, menor densidad del tallo, mayor nitrógeno foliar y una fuerte tendencia de mayores tasas fotosintéticas saturadas de luz. Estos atributos de la estrategia de vida rápida se alinean tanto con las observaciones basadas en el campo como con los resultados experimentales de sequía. Para sobrevivir a la sequía, C. villosissimus es caducifolia facultativa, perdiendo hojas inferiores durante la estación seca y creciendo rápidamente hojas nuevas en la estación húmeda. CONCLUSIONES: Revelamos una estrategia de adaptación a la sequía que, hasta donde sabemos, no ha sido documentada previamente en hierbas tropicales. Esta adaptación divergente a la sequía evolucionó recientemente y es un componente importante del aislamiento reproductivo entre C. villosissimus y C. allenii, lo que indica que los cambios adaptativos para sobrevivir a la sequía estacional pueden ser un eje subestimado de la diversificación de las plantas del sotobosque neotropical.

Outer surface protein polymorphisms linked to host-spirochete association in Lyme borreliae.

Lyme borreliosis is caused by multiple species of the spirochete bacteria Borrelia burgdorferi sensu lato. The spirochetes are transmitted by ticks to vertebrate hosts, including small- and medium-sized mammals, birds, reptiles, and humans. Strain-to-strain variation in host-specific infectivity has been documented, but the molecular basis that drives this differentiation is still unclear. Spirochetes possess the ability to evade host immune responses and colonize host tissues to establish infection in vertebrate hosts. In turn, hosts have developed distinct levels of immune responses when invaded by different species/strains of Lyme borreliae. Similarly, the ability of Lyme borreliae to colonize host tissues varies among different spirochete species/strains. One potential mechanism that drives this strain-to-strain variation of immune evasion and colonization is the polymorphic outer surface proteins produced by Lyme borreliae. In this review, we summarize research on strain-to-strain variation in host competence and discuss the evidence that supports the role of spirochete-produced protein polymorphisms in driving this variation in host specialization. Such information will provide greater insights into the adaptive mechanisms driving host and Lyme borreliae association, which will lead to the development of interventions to block pathogen spread and eventually reduce Lyme borreliosis health burden.

Ecological differentiation and local adaptation in two sister species of Neotropical Costus (Costaceae).

Reciprocal transplant experiments have often provided evidence of local adaptation in temperate plants, but few such studies have been conducted in the tropics. To enhance our knowledge of local adaptation in tropical plants, we studied natural populations of two recently diverged Neotropical plant species, Costus allenii and C. villosissimus, in central Panama. We found that these species display a parapatric distribution that reflects local environmental differences on a fine geographic scale: C. allenii is found along ravines in the understory of primary forest, while C. villosissimus is found along forest edges. Light availability was lower in C. allenii habitats, while precipitation and soil moisture were lower in C. villosissimus habitats. We carried out reciprocal transplant experiments with seeds and clones of mature plants to test the hypothesis that the parapatric distribution of these species is due to divergent adaptation to their local habitats. We found strong evidence of local adaptation, i.e., when grown in their "home" sites, each species outperformed the species from an "away" site. Our finding that C. allenii and C. villosissimus are mainly isolated by their microhabitats provides a first step toward understanding the mechanisms of adaptation and speciation in the tropics.

Unification of methods for estimating the strength of reproductive isolation.

Understanding the evolution of reproductive isolation is tantamount to describing the origin of species. Therefore, a primary goal in evolutionary biology is to identify which reproductive barriers are most important to the process. To achieve this goal, the strength of multiple forms of isolation must be compared in an equivalent manner. However, a diversity of methods has been used to estimate barrier strength, falling into several mathematically distinct categories. This study provides a unified method for calculating isolation that relates the amount of gene flow experienced by taxa to random expectations in a simple linear framework. This approach has three distinct advantages over previous methods: (1) it is directly related to gene flow, (2) it is symmetrical, such that measures in both the positive and negative range are comparable, and (3) it is equivalent between broad categories of reproductive isolation, allowing for appropriate comparisons. This linear formulation can be adjusted for use in all forms of isolation, and can accommodate cases in which null expectations for con- and heterospecific gene flow differ. Additionally, this framework can be used to calculate total reproductive isolation and the relative contributions of individual barriers.

The biology of speciation.

Since Darwin published the "Origin," great progress has been made in our understanding of speciation mechanisms. The early investigations by Mayr and Dobzhansky linked Darwin's view of speciation by adaptive divergence to the evolution of reproductive isolation, and thus provided a framework for studying the origin of species. However, major controversies and questions remain, including: When is speciation nonecological? Under what conditions does geographic isolation constitute a reproductive isolating barrier? and How do we estimate the "importance" of different isolating barriers? Here, we address these questions, providing historical background and offering some new perspectives. A topic of great recent interest is the role of ecology in speciation. "Ecological speciation" is defined as the case in which divergent selection leads to reproductive isolation, with speciation under uniform selection, polyploid speciation, and speciation by genetic drift defined as "nonecological." We review these proposed cases of nonecological speciation and conclude that speciation by uniform selection and polyploidy normally involve ecological processes. Furthermore, because selection can impart reproductive isolation both directly through traits under selection and indirectly through pleiotropy and linkage, it is much more effective in producing isolation than genetic drift. We thus argue that natural selection is a ubiquitous part of speciation, and given the many ways in which stochastic and deterministic factors may interact during divergence, we question whether the ecological speciation concept is useful. We also suggest that geographic isolation caused by adaptation to different habitats plays a major, and largely neglected, role in speciation. We thus provide a framework for incorporating geographic isolation into the biological species concept (BSC) by separating ecological from historical processes that govern species distributions, allowing for an estimate of geographic isolation based upon genetic differences between taxa. Finally, we suggest that the individual and relative contributions of all potential barriers be estimated for species pairs that have recently achieved species status under the criteria of the BSC. Only in this way will it be possible to distinguish those barriers that have actually contributed to speciation from those that have accumulated after speciation is complete. We conclude that ecological adaptation is the major driver of reproductive isolation, and that the term "biology of speciation," as proposed by Mayr, remains an accurate and useful characterization of the diversity of speciation mechanisms.