Evolution of Pollen in Asexual Plants
Title: Evolution of Pollen in Asexual Plants
DNr: NAISS 2024/22-88
Project Type: NAISS Small Compute
Principal Investigator: Yannick Woudstra <yannick.woudstra@su.se>
Affiliation: Stockholms universitet
Duration: 2024-01-26 – 2025-02-01
Classification: 10615


Pollination is one of the most intricate mutualisms in nature. Many plants provide nutritious pollen grains to reward feeding insects for their pollination services. Plants carefully balance male reproductive fitness gains against the energetic burden of producing nutritious pollen that ends up “wasted” in insect stomachs. Common dandelions (Taraxacum officinale) produce abundant pollen but are likely not reaping the reproductive benefits as most of them are asexual (apomictic), reproducing independent of pollen. Decreased selection on nutritious pollen in the absence of sex is therefore expected to lead to less and more degenerate (poor) pollen. This has potentially disastrous consequences for the feeding insects, who depend on the pollen for their own survival and reproduction. Up to 80% of the early spring pollen food source is provided by dandelions. Understanding how pollen degenerates under the loss of sex is therefore of paramount importance to predicting the development of an essential insect food source. This is especially significant in Northern European countries, such as Sweden, where sexual dandelions are absent. The evolutionary degeneration of pollen under asexuality will be studied from a phenotypic and genomic viewpoint using comparative transcriptomics of sexual and asexual dandelions. We will use verified collections of sexual diploid and asexual triploid dandelions, grown in a common environment. In addition, we will generate novel asexual apomicts by making controlled crosses of sexual and asexual dandelions to control for direct effects of ploidy change on pollen gene expression. With RNAseq we will determine the gene expression patterns of mature pollen grains in the transition from sexual => novel asexual => old widespread asexual. Crosses between sexual and asexual dandelions occur naturally in areas of co-occurring sexual diploids and asexual triploids. A diploid pollen grain from an asexual plant can fertilise a haploid egg cell of a diploid plant, resulting in asexual triploid offspring. We will use available seed collections of such crosses that were produced in a controlled environment by hand-pollinating verified diploid plants with pollen from verified triploid plants. The asexuality of the offspring has been confirmed with ploidy analysis using flow cytometry. Seeds of sexual, novel asexual and old asexual dandelions will then be germinated simultaneously, and seedlings will be cultivated in the same greenhouse conditions. Pollen will be harvested from one inflorescence per plant on the first day the inflorescence opens. Immediately afterwards, we will harvest flower petal and leaf material of the same plant for comparative expression patterns in the same plant. Three distinct accessions for each category will be used to provide biological replicates. The pollen RNAseq data will be compared to the expression patterns of flower petals and leaves of the same plants to identify genes with pollen-biased expression. Comparison to the annotated reference genome of a diploid sexual dandelion will reveal which genes are essential for pollen development. In addition, we will determine whether relaxed selection on these pollen development genes leads to mutational degeneration in old asexual lines of dandelions.