Dynamics of apomictic and sexual reproduction during primary succession on a glacier forefield in the Swiss Alps

Christian Sailer,Jürg Stöcklin,Ueli Grossniklaus

doi:10.1038/s41598-020-64367-9

Christian Sailer, Jürg Stöcklin + Show 1 more

Open Access

https://doi.org/10.1038/s41598-020-64367-9

Copy DOI

Abstract

Apomixis, the asexual reproduction through seeds, is thought to provide reproductive assurance when ploidy is not even and/or when population density is low. Therefore, apomicts are expected to be more abundant, and the frequency of apomictic offspring higher, at early stages of primary succession when mates are rare. To test this hypothesis, we sampled facultative apomictic Hieracium pilosella L. along the successional gradient on a glacier forefield and determined their ploidy, the level of apomixis in their offspring, and the genetic diversity of the entire meta-population and within subpopulations. We found that apomixis is more common in odd- and aneuploid cytotypes, which are more frequent at early stages of primary succession. However, apomixis was uncommon at all successional stages and sexual hexaploids were dominating throughout. Reproductive assurance was reflected in the higher fertility of all odd-ploid apomictic plants (3×, 5×) by avoiding meiosis, illustrating that apomixis provides an escape from sterility, as proposed by Darlington. Odd-ploid plants are supposedly better colonizers (Baker’s law), which is supported by their higher occurrence close to the glacier snout. Independent of succession, we found gene flow between apomicts and sexuals, which allows for the continuous creation of new apomictic and sexual genotypes. We conclude that apomixis in H. pilosella does indeed provide an escape from sterility, and therefore reproductive assurance, in aneuploid cytotypes. We further propose that apomixis preserves beneficial combinations of unlinked alleles in every generation for as long as apomictic genotypes persist in the population.

Highlights

Apomixis can be viewed as a deregulation of sexual processes, resulting in asexual reproduction through seeds[1,2,3,4]
As new apomictic genotypes arise from sexual reproduction, apomixis is not lost as a trait
The same is true if only LOSS OF PARTHENOGENESIS (LOP) is present, leading to offspring with reduced ploidy (n + 0, polyhaploid, Fig. 1), which is the result of meiosis and parthenogenesis, a sexual and an apomictic process, respectively

Summary

Introduction

Apomixis can be viewed as a deregulation of sexual processes, resulting in asexual reproduction through seeds[1,2,3,4]. No mechanism exists to purge deleterious mutations from the genomic pool of a population This results in a successive reduction in fitness and, eventually, genotypes that have reached a critical threshold of deleterious mutations go extinct, a process known as Muller’s ratchet[18,19]. The same is true if only LOP is present, leading to offspring with reduced ploidy (n + 0, polyhaploid, Fig. 1), which is the result of meiosis and parthenogenesis, a sexual and an apomictic process, respectively. If both loci are absent, sexual reproduction occurs, leading to n + n offspring (Fig. 1). H. pilosella provides a system in which we have a good understanding of the genetic basis of apomixis and the formation of different cytotypes, allowing inferences about the processes that led to the formation of a specific individual

Methods

Results

Discussion

Conclusion