Marianne Kivastik, University of Tartu, discusses her article: A pan-European citizen science study reveals factors related to biased morph ratios in the heterostylous plant Primula veris
Human-induced environmental changes have many negative consequences for biodiversity on all levels. Among plants, insect-pollinated species can be especially affected by habitat loss and fragmentation because of the adverse effects on pollinators. Heterostyly is a reproductive system characterised by the occurrence of multiple floral morphs within a population to ensure outcrossing by pollinators. Specifically, these morphs vary in the relative lengths of both the style and stamens. Habitat loss, reduced plant population sizes, and climate change have raised concerns about the impact of these factors on morph ratios and the related consequences for the fitness of heterostylous species.
The cowslip (Primula veris) has long served as a model species for exploring the consequences of landscape changes, as well as for molecular and evolutionary studies on heterostyly. In our study, with the help of citizen scientists, we studied the floral morph ratios of thousands of cowslip populations in response to plant population size, landscape context, and climatic factors all over Europe. As a result of extensive communication efforts with co-authors from 16 countries, we got data from over 3,000 populations and about 350,000 plant individuals. Communication efforts included (among many other things) making a web platform with all the information and instructions for carrying out the observations as well as a form through which participants could easily send us the data (check out https://cowslip.science/en).
Heterostylous plant populations are expected to have equal frequencies (i.e, isoplethy) of each morph type to ensure optimal fertilisation and reproduction. Unexpectedly, our results showed a substantial morph deviation from isoplethy and a significant excess (9%) of S-morphs. Also, the deviation of morph frequency from equilibrium (not affected by morph identity) was substantially stronger in smaller populations. So far, such substantial dominance of one morph has not been shown. If anything, theory predicts there should be more L-morphs because of their higher ability to self-fertilise within the same morph type, and thus produce more L-morphs. We specifically checked which landscape and climatic factors could support the prevalence of S-morphs and interestingly found that higher summer precipitation and land use intensity were associated with an increased prevalence of S-morphs.
Higher precipitation can affect the morph frequencies through plant-pollinator interactions, and higher humidity can affect pollen viability. A study on closely related Primula palinuri suggests that L-morph pollen is more sensitive to higher humidity levels, which can lead to higher survival of S-morph pollen. A higher proportion of built-up areas affected both overall morph balance and the prevalence of S-morphs. This further confirms the negative consequences of human impact on heterostylous species.
Additionally, we obtained some observations of homostylous individuals, which are so far rather rare for cowslips. Leaf samples of these individuals were sent for further analysis and genotyping, which revealed that those individuals have probably risen from mutations in the L-morphs, contrary to the known origin of Primula vulgaris’ homostyles. As a close relative to P. vulgaris, we would have expected to find the same origin for P. veris’ homostyles. Because our genotyping results only confirmed no S-morph genotype, the origin of homostylous P. veris needs to be further studied.
Our study raised yet many new questions regarding the exact mechanisms behind the factors favouring S-morph dominance, the role of plant-pollinator interactions, and the origin of homostylous cowslip individuals. Thus, we definitely have a lot of new study questions for our future work and hope that our study inspires other researchers studying heterostylous species or other insect-pollinated plants. We also recommend the use of citizen science to connect with nature enthusiasts and gather data at large scale that is unattainable by scientists alone.
