Transcriptome profiling and comparison of Rhinanthus major and Rhinanthus minor reciprocal F1 hybrids during seed stratification and germination

Germination is a vital stage in a plants life cycle, and a different germination behavior of offspring in comparison to their parents can have fitness consequences. In studies on hybridization between Rhinanthus minor and R. major, low germination rates of F1 hybrids with R. major as the maternal parent have often been reported. In contrast, the F1m hybrid, with R. minor as the maternal parent, germinates readily and rapidly. In order to find the cause of this difference, we used RNA-Seq to obtain transcriptome profiles of F1a and F1m seeds during stratification at 4C and just after germination, after 40 days of stratification for the F1m seeds and 60 days for the F1a seeds. A comparison of the transcriptome of F1a seeds that had just germinated (60 days) with non-germinated F1a seeds after 40 and 60 days revealed 2918 and 1349 differentially expressed (DE) genes, respectively. For F1m seeds, 958 genes showed differential expression in germinated and non-germinated seeds after 40 days. The DE genes of F1a and F1m hybrids clustered into two separate groups, even though they had the same parents, and no differentially expression was found for plastid genes. Non-germinated F1a seeds had an abundance of enzymes and proteins associated with peroxidase activity, peroxiredoxin activity and nutrient reservoir activity. Expression of genes related to seed germination and seed development increased in non-germinated F1a hybrid seeds between 40 and 60 days of cold stratification. F1a seeds that had germinated showed an upregulation of genes related to the gibberellic acid-mediated signaling pathway and response to gibberellin, along with a low expression of DELLA superfamily. Although the results demonstrated strong differences in gene expression during stratification between the reciprocal hybrids, we could not identify its cause, since no plastid genes were differentially expressed. It is possible that differences in embryo development after seed formation and before stratification play a role, including epigenetic imprinting.

Fitness of reciprocal F1 hybrids between Rhinanthus minor and Rhinanthus major under controlled conditions and in the field

The performance of first-generation hybrids determines to a large extent the long-term outcome of hybridization in natural populations. F1 hybrids can facilitate further gene flow between the two parental species, especially in animal-pollinated flowering plants. We studied the performance of reciprocal F1 hybrids between Rhinanthus minor and R. major, two hemiparasitic, annual, self-compatible plant species, from seed germination to seed production under controlled conditions and in the field. We sowed seeds with known ancestry outdoors before winter and followed the complete life cycle until plant death in July the following season. While germination under laboratory conditions was much lower for the F1 hybrid formed on R. major compared to the reciprocal hybrid formed on R. minor, this difference disappeared under field conditions, pointing at an artefact caused by the experimental conditions during germination in the lab rather than at an intrinsic genetic incompatibility. Both F1 hybrids performed as well as or sometimes better than R. minor, which had a higher fitness than R. major in one of the two years in the greenhouse and in the field transplant experiment. The results confirm findings from naturally mixed populations, where F1 hybrids appear as soon as the two species meet and which leads to extensive advanced-hybrid formation and introgression in subsequent generations.

Insect Herbivory on the Plant Rhinanthus minor over its Elevational Range in the Canadian Rockies

All species of plants and animals occur over a finite area of the Earth’s surface. This is referred to as the species range, and many species ranges have shifted or are predicted to shift with climate change. Scientific models have predicted how these shifts are expected to change and what proportion of the implicated species will go extinct in the process. Most models assume that climatic variables such as temperature and rainfall are solely responsible for these range shifts. However, we know that the success of a species is strongly influenced by both their positive and negative interactions with other species, such as competition, mutualism, predation and herbivory. But how these biotic factors affect species ranges is poorly understood. I am using a field experiment on a species in its native habitat to better understand these interactions. My study took place in the Canadian Rocky Mountains on populations of the plant Yellow Rattle (Rhinanthus minor). I studied two transects, each with plant populations at low, mid and high elevations. Insect herbivory on plant populations was observed, as well as manipulated, via a pesticide treatment to reduce insect herbivory, and a clipping treatment to simulate natural insect herbivory. Understanding herbivory and herbivore-plant interactions over an elevational gradient may help give us a clearer idea of the complex relationship between the climatic and biotic factors that affect plant species ranges.

Competitive ability of Rhinanthus minor L. in relation to productivityin the Rengen Grassland Experiment

Rhinanthus minor (yellow-rattle) can be used for restoration of species-rich grasslands but is vulnerable to competitive exclusion from high total aboveground biomass production of vascular plants. We asked (1) whether there is a threshold limit for total annual aboveground biomass production of vascular plants above which R. minor cannot establish viable population in grasslands and (2) how is cover of R. minor in grassland related to standing biomass of bryophytes. Data were collected in the Rengen Grassland Experiment (RGE) established in Germany in 1941 with following fertilizer treatments: unfertilized control, application of Ca, CaN, CaNP, CaNPKCl and CaNPK₂SO₄. Cover of R. minor and total annual aboveground biomass production of vascular plants were determined from 2005 to 2009. Further relationship between standing biomass of bryophytes and cover of R. minor was analyzed in 2006. Mean cover of R. minor over five years ranged from 0.7% to 12.3% in CaNPK₂SO₄ and control treatments, respectively. Cover of R. minor was significantly negatively related to total annual aboveground biomass production of vascular plants and cover of R. minor was below 3% in all plots with total annual aboveground dry matter biomass production of vascular plants higher than 5 t/ha. Although cover of R. minor was markedly reduced in highly productive plots in the RGE, high standing biomass of bryophytes (1.8 t/ha) in low productive control was not an obstacle for establishment of its viable population. We concluded, that viable population of R. minor can be established in grasslands only if total annual aboveground dry matter biomass production of vascular plants is below 5 t/ha regardless on standing biomass of bryophytes.