Equipped for Migrations Across High Latitude Regions? Reduced Spur Length and Outcrossing Rate in a Biennial Halenia elliptica (Gentianaceae) With Mixed Mating System Along a Latitude Gradient

Clinal variation in quantitative traits but not in evolutionary potential along elevational and latitudinal gradients in the widespread Anthyllis vulneraria

PREMISE

Strong elevational and latitudinal gradients allow the study of genetic differentiation in response to similar environmental changes. However, it is uncertain whether the environmental changes along the two types of gradients result in similar genetically based changes in quantitative traits. Peripheral arctic and alpine populations are thought to have less evolutionary potential than more central populations do.

METHODS

We studied quantitative traits of the widespread Anthyllis vulneraria in a common garden. Plants originated from 20 populations along a 2000-m elevational gradient from the lowlands to the elevational limit of the species in the Alps, and from 20 populations along a 2400-km latitudinal gradient from the center of the distribution of the species in Central Europe to its northern distributional margin.

RESULTS

Most traits showed similar clinal variations with elevation and latitude of origin, and the magnitude of all measured traits in relation to mean annual temperature was similar. Higher QST values than FST values in several traits indicated diversifying selection, but for others QST was smaller than FST. Genetic diversity of quantitative traits and neutral molecular markers was not correlated. Plasticity in response to favorable conditions declined with elevation and less strongly with latitude of origin, but the evolvability of traits did not.

CONCLUSIONS

The clinal variation suggests adaptive differentiation of quantitative traits along the two gradients. The evolutionary potential of peripheral populations is not necessarily reduced, but lower plasticity may threaten their survival under rapidly changing climatic conditions.

Latitudinal trends in mating system traits in the highly self-fertilizing Lobelia inflata revealed by community science

Mating systems in angiosperms range from obligate outcrossing to highly self-fertilizing. The belief that obligate selfing does not exist is contradicted by genetic evidence in several populations of L. inflata, in which selfing is enforced by the anthers enclosing the style. However, whether the mating systems of these populations are typical, or an extreme across the species range is unknown. Such trends are hypothesized to result from selection for reproductive assurance under mate limitation at range margins. Here, we use ~7500 iNaturalist community science images, in which stylar exsertion can be observed, to test this hypothesis in L. inflata and, for comparison, in four typical congeneric Lobelias that express a staminate, then a pistillate phase (protandry). Specifically, we analyzed the effects of latitude and range marginality on the frequency of stylar exsertion and number of exserted flowers. Outcrossing capacity in L. inflata increased at low latitudes and near the overall range center, supporting our hypothesis, with exsertion frequencies significantly lower than in congenerics. Interestingly, in outcrossing capable individuals, the number of style-exserted flowers was consistent across the species range and among species, indicating outcrossing capable L. inflata individuals resemble congenerics. These findings suggest that variation in stylar exsertion is expressed among individuals rather than by all individuals within populations. However, whether this is a result of differences in exsertion allele frequencies or of differentiation in the induction of a threshold trait requires further study. Moreover, the trends in outcrossing capability revealed here imply the potential for geographic variation in L. inflata mating system.

Phenotypic Selection in Halenia elliptica D. Don (Gentianaceae), an Alpine Biennial with Mixed Mating System

The transition from outcrossing to selfing is a common evolutionary trend in flowering plants, and floral traits change significantly with the evolution of selfing. Whether or not plant traits are subjected to selection remains an open question in species with mixed mating systems. We examined phenotypic selection in two populations of Halenia elliptica with different selfing rates. We found that the pollen–ovule ratio, seed size, plant height, spur length, and pollinator visitation rate in the population with the higher selfing rate were lower than those in the population with the lower selfing rate. Selfing provides reproductive assurance for populations when pollinator service is low, and the floral traits that are associated with selfing syndrome are evident in populations with a higher selfing rate but are subjected to weak selection in each of the two populations with different selfing rates. Directional selection for an early flowering time indicated that late blooming flowers could experience a risk of seed development in alpine environments, and for large plants, selection indicated that seed production could be limited by the available resources. The floral traits that are associated with pollinator attraction and specialization could be subjected to weak selection at the plant level as selfing evolves, and the selective pressures that are independent of pollinators might not change significantly; highlighting the selective biotic and abiotic pressures that shape the morphological traits of plant species and their independence from the mating system.

Multiple lines of evidence supports the two varieties of Halenia elliptica (Gentianaceae) as two species

Delimiting species requires multiple sources of evidence. Here, we delimited two varieties of Halenia elliptica (Gentianaceae) using several lines of evidence, including morphological traits and mating system in a sympatric population, phylogenetic relationships based on nrITS and cpDNA (rpl16) data, and complete chloroplast genome sequences. Comparative analysis of 21 morphological traits clearly separates the two varieties of H. elliptica. Examination of the flowering process and pollination treatments indicate that H. elliptica var. grandiflora produces seeds via outcrossing, whereas H. elliptica var. elliptica produces seeds via mixed mating. Furthermore, hand-pollinated hybridization of the two varieties produced no seeds. Observations of pollinators showed that when bees began a pollination bout on H. elliptica var. grandiflora they preferred to continue pollinating this variety; however, when they began a pollination bout on H. elliptica var. elliptica, they showed no preference for either variety. Phylogenetic analysis confirmed the monophyly of H. elliptica, which was further divided into two monophyletic clades corresponding to the two varieties. A large number of variants from the chloroplast genomes reflected remarkable genetic dissimilarities between the two varieties of H. elliptica. We recommend that the two varieties of H. elliptica should be revised as two species (H. elliptica and H. grandiflora). Our findings indicate that H. elliptica varieties may have split into two separate species due to a shift in mating system, changes in flowering phenology and/or post-pollination reproductive isolation.

Cell number explains the intraspecific spur-length variation in an Aquilegia species

Variations of nectar spur length allow pollinators to utilize resources in novel ways, leading to the different selective pressures on spurs and allowing taxa to diversify. However, the mechanisms underlying spur length variation remain unclear. Interspecific comparisons of spur length suggest that both cell division and anisotropic expansion could explain the changes of spur length, and that hormone-related genes contribute to the process of spur formation. In contrast, little is known about intraspecific spur length variation. In Aquilegia rockii, spur length varies strikingly, ranging from 1 mm to 18 mm. To examine the potential mechanisms underlying spur length variation in A. rockii, we observed cell morphology and analyzed RNA-seq of short- and long-spurred flowers. Scanning electron microscopy revealed that at two positions on spurs there were no differences in either cell density or cell anisotropy between short- and long-spurred flowers, suggesting that in A. rockii changes in cell number may explain variations in spur length. In addition, we screened transcriptomes of short- and long-spurred flowers for differentially expressed genes; this screen identified several genes linked to cell division (e.g., F-box, CDKB2-2, and LST8), a finding which is consistent with our analysis of the cellular morphology of spurs. However, we did not find any highly expressed genes involved in the hormone pathway in long-spurred flowers. In contrast to previous hypotheses that anisotropic cell expansion leads to interspecific spur variation in Aquilegia, our results suggest that cell number changes and related genes are mainly responsible for spur length variations of A. rockii. Furthermore, the underlying mechanisms of similar floral traits in morphology may be quite different, enriching our understanding of the mechanisms of flower diversity in angiosperms.

Pollen limitation and autonomous selfing ability interact to shape variation in outcrossing rate across a species range

PREMISE

Hermaphroditic plants commonly reproduce through a mixture of selfing and outcrossing. The degree to which outcrossing rates reflect the availability of outcross pollen, genetic differentiation in the ability to autonomously self-fertilize, or both is often unclear. Despite the potential for autonomy and the pollination environment to jointly influence outcrossing, this interaction is rarely studied.

METHODS

We reviewed studies from the literature that tested whether the pollination environment or floral traits that cause autonomous selfing predict variation in outcrossing rate among populations. We also measured outcrossing rates in 23 populations of Campanula americana and examined associations with the pollination environment, autonomy, and their interaction.

RESULTS

Our review revealed that traits that facilitate selfing were often negatively associated with outcrossing rates whereas most aspects of the pollination environment poorly predicted outcrossing. Populations of C. americana varied from mixed mating to highly outcrossing, but variation was unrelated to population size, density, pollen limitation, or autonomous selfing ability. Outcrossing rate was significantly influenced by an interaction between autonomous selfing ability and pollen limitation. Across highly autonomous populations, elevated pollen limitation was associated with reduced outcrossing, while there was no relationship for less autonomous populations.

CONCLUSIONS

Both the ability to self autonomously and pollen limitation interact to shape outcrossing rates in C. americana. This work suggests that autonomy affords mating-system flexibility, though it is not ubiquitous in all populations across the species range. Interactions between traits influencing autonomy and pollen limitation are likely to explain variation in outcrossing rates among populations of flowering plants.