Genome-scale data resolves the timing of divergence in Joshua trees.

PREMISE: Joshua trees (Yucca brevifolia and Y. jaegeriana) and their yucca moth pollinators (Tegeticula synthetica and T. antithetica) are a model system for studies of plant-pollinator coevolution and, they are thought to be one of the only cases in which there is compelling evidence for cospeciation driven by coevolution. Previous work attempted to evaluate whether divergence between the plant and their pollinators was contemporaneous. That work concluded that the trees diverged more than 5 million years ago-well before the pollinators. However, clear inferences were hampered by a lack of data from the nuclear genome and low genetic variation in chloroplast genes. As a result, divergence times in the trees could not be confidently estimated. METHODS: We present an analysis of whole chloroplast genome sequence data and RADseq data from >5000 loci in the nuclear genome. We developed a molecular clock for the Asparagales and the Agavoideae using multiple fossil calibration points. Using Bayesian inference, we produced new estimates for the age of the genus Yucca and for Joshua trees. We used calculated summary statistics describing genetic variation and used coalescent-based methods to estimate population genetic parameters. RESULTS: We find that the Joshua trees are moderately genetically differentiated, but that they diverged quite recently (~100-200 kya), and much more recently than their pollinators. CONCLUSIONS: The results argue against the notion that coevolution directly contributed to speciation in this system, suggesting instead that coevolution with pollinators may have reinforced reproductive isolation following initial divergence in allopatry.

Mitochondrial Divergence between Western and Eastern Great Bustards: Implications for Conservation and Species Status.

The great bustard is the heaviest bird capable of flight and an iconic species of the Eurasian steppe. Populations of both currently recognized subspecies are highly fragmented and critically small in Asia. We used DNA sequence data from the mitochondrial cytochrome b gene and the mitochondrial control region to estimate the degree of mitochondrial differentiation and rates of female gene flow between the subspecies. We obtained genetic samples from 51 individuals of Otis tarda dybowskii representing multiple populations, including the first samples from Kazakhstan and Mongolia and samples from near the Altai Mountains, the proposed geographic divide between the subspecies, allowing for better characterization of the boundary between the 2 subspecies. We compared these with existing sequence data (n = 66) from Otis tarda tarda. Our results suggest, though do not conclusively prove, that O. t. dybowskii and O. t. tarda may be distinct species. The geographic distribution of haplotypes, phylogenetic analysis, analyses of molecular variance, and coalescent estimation of divergence time and female migration rates indicate that O. t. tarda and O. t. dybowskii are highly differentiated in the mitochondrial genome, have been isolated for approximately 1.4 million years, and exchange much less than 1 female migrant per generation. Our findings indicate that the 2 forms should at least be recognized and managed as separate evolutionary units. Populations in Xinjiang, China and Khövsgöl and Bulgan, Mongolia exhibited the highest levels of genetic diversity and should be prioritized in conservation planning.

Floral scent of Joshua trees (Yucca brevifolia sensu lato): Divergence in scent profiles between species but breakdown of signal integrity in a narrow hybrid zone.

PREMISE OF THE STUDY: The role of floral scent in facilitating reproductive isolation between closely related plants remains poorly understood. Yucca brevifolia and Yucca jaegeriana are pollinated by different moth species in allopatry, but in a narrow contact zone, pollinator-host specificity breaks down, resulting in hybridization between species. We explored the chemical basis for reproductive isolation and hybridization in these Joshua trees by characterizing the floral scent of each species in allopatry, analyzing scent profiles from trees in the contact zone, and matching these data with genotypic and phenotypic data. METHODS: We analyzed floral volatiles using gas chromatography-mass spectrometry, tested for species divergence of scent profiles and classified trees in the contact zone as hybrid or either parental species. We used floral and vegetative morphological data and genotypic data to classify trees and analyzed whether certain trait combinations were more correlated than others with respect to assignment of trees and whether frequencies of classified tree types differed depending on which data set was used. KEY RESULTS: The Joshua tree floral scent included oxygenated 8-carbon compounds not reported for other yuccas. The two species differed (P < 0.001) in scent profiles. In the contact zone, many hybrids were found, and phenotypic traits were generally weakly correlated, which may be explained by extensive gene flow between species or by exposure to different selection pressures. CONCLUSIONS: Although the two Joshua tree species produce distinct floral scent profiles, it is insufficient to prevent attraction of associated pollinators to both hosts. Instead, floral morphology may be the key trait mediating gene flow between species.

Asymmetric hybridization and gene flow between Joshua trees (Agavaceae: Yucca) reflect differences in pollinator host specificity.

The angiosperms are by far the largest group of terrestrial plants. Their spectacular diversity is often attributed to specialized pollination. Obligate pollination mutualisms where both a plant and its pollinator are dependent upon one another for reproduction are thought to be prone to rapid diversification through co-evolution and pollinator isolation. However, few studies have evaluated the degree to which pollinators actually mediate reproductive isolation in these systems. Here, we examine evidence for hybridization and gene flow between two subspecies of Joshua tree (Yucca brevifolia brevifolia and Yucca brevifolia jaegeriana) pollinated by two sister species of yucca moth. Previous work indicated that the pollinators differ in host specificity, and DNA sequence data suggested asymmetric introgression between the tree subspecies. Through intensive sampling in a zone of sympatry, a large number of morphologically intermediate trees were identified. These included trees with floral characters typical of Y. b. jaegeriana, but vegetative features typical of Y. b. brevifolia. The opposite combination-Y. b. brevifolia flowers with Y. b. jaegeriana vegetative morphology-never occurred. Microsatellite genotyping revealed a high frequency of genetically admixed, hybrid trees. Coalescent-based estimates of migration indicated significant gene flow between the subspecies and that the direction of gene flow matches differences in pollinator host fidelity. The data suggest that pollinator behaviour determines the magnitude and direction of gene flow between the two subspecies, but that specialized pollination alone is not sufficient to maintain species boundaries. Natural selection may be required to maintain phenotypic differences in the face of ongoing gene flow.

Weak polygyny in California sea lions and the potential for alternative mating tactics.

Female aggregation and male territoriality are considered to be hallmarks of polygynous mating systems. The development of genetic parentage assignment has called into question the accuracy of behavioral traits in predicting true mating systems. In this study we use 14 microsatellite markers to explore the mating system of one of the most behaviorally polygynous species, the California sea lion (Zalophus californianus). We sampled a total of 158 female-pup pairs and 99 territorial males across two breeding rookeries (San Jorge and Los Islotes) in the Gulf of California, Mexico. Fathers could be identified for 30% of pups sampled at San Jorge across three breeding seasons and 15% of sampled pups at Los Islotes across two breeding seasons. Analysis of paternal relatedness between the pups for which no fathers were identified (sampled over four breeding seasons at San Jorge and two at Los Islotes) revealed that few pups were likely to share a father. Thirty-one percent of the sampled males on San Jorge and 15% of the sampled males on Los Islotes were assigned at least one paternity. With one exception, no male was identified as the father of more than two pups. Furthermore, at Los Islotes rookery there were significantly fewer pups assigned paternity than expected given the pool of sampled males (p<0.0001). Overall, we found considerably lower variation in male reproductive success than expected in a species that exhibits behavior associated with strongly polygynous mating. Low variation in male reproductive success may result from heightened mobility among receptive females in the Gulf of California, which reduces the ability of males to monopolize groups of females. Our results raise important questions regarding the adaptive role of territoriality and the potential for alternative mating tactics in this species.

Characterization of microsatellite loci in Yucca brevifolia (Agavaceae) and cross-amplification in related species.

PREMISE OF THE STUDY: Microsatellite primers were characterized in Yucca brevifolia for use in population genetic studies and, particularly, analyses of gene flow between varieties. METHODS AND RESULTS: We characterized 12 microsatellite loci polymorphic in Yucca brevifolia by screening primers that were developed using an SSR-enriched library or which were previously described in Yucca filamentosa. Genetic analysis of four populations resulted in the mean number of alleles per locus ranging from 10.25 to 14.58 and mean expected heterozygosity from 0.78 to 0.88. Cross-amplification of all 12 loci was attempted in six additional yucca species. CONCLUSIONS: These loci should prove useful for population genetic research in Yucca brevifolia, and cross-amplification of these loci in related species suggests that they may be useful in studies of hybridization and introgression between species.

First evidence for adoption in California sea lions.

Demographic parameters such as birth and death rates determine the persistence of populations. Understanding the mechanisms that influence these rates is essential to developing effective management strategies. Alloparental behavior, or the care of non-filial young, has been documented in many species and has been shown to influence offspring survival. However, the role of alloparental behavior in maintaining population viability has not been previously studied. Here, we provide the first evidence for adoption in California sea lions and show that adoption potentially works to maintain a high survival rate of young and may ultimately contribute to population persistence. Alloparental behavior should have a positive effect on the population growth rate when the sum of the effects on fitness for the alloparent and beneficiary is positive.