The Evolutionary Dynamics of Apomixis in Ferns: A Case Study from Polystichoid Ferns

Abstract

The disparate distribution of apomixis between the major plant lineages is arguably one of the most paradoxical phenomena in plant evolution. Ferns are particularly interesting for addressing this issue because apomixis is more frequent than in any other group of plants. Here, we use a phylogenetic framework to explore some aspects of the evolution of apomixis in ferns and in particular in the polystichoid ferns. Our findings indicate that apomixis evolved several times independently in three different clades of polystichoid ferns. A lineage-wide perspective across ferns indicates a correlation between apomixis and the species richness of lineages; however BiSSE tests did not recover evidence for a correlation of apomixis and diversification rates. Instead, evidence was recovered supporting an association between the establishment of apomixis and reticulate evolution, especially in the establishment of triploid hybrids. Diversification time estimates supported the hypothesis of short living apomictic lineages and indicated a link between the establishment of apomixis and the strengthening of the monsoons caused by the lifting of the Qinghai-Tibetan plateau. In general our results supported the hypothesis for the rare establishment of apomictic lineages, high extinction risks, and low speciation rates.

Sex and the Single Gametophyte: Revising the Homosporous Vascular Plant Life Cycle in Light of Contemporary Research

“…Diversity in gametophyte form and physiology has enabled ferns to radiate into the wide range of habitats they must first colonize through gametophyte establishmenthabitats including dark, moist forest floors; extremely bright, exposed forest canopies; tropical regions never experiencing at AIBS on November 18, 2016 http://bioscience.oxfordjournals.org/ Downloaded from freezing temperatures; the coldest of northern latitudes and mountain tops where gametophytes formed from spores released in late summer must overwinter; and rocky desert extremes where gametophytes produce sporophytes despite the rare presence of environmentally available water (Liu et al 2012). Some gametophytes tolerate environmental extremes exceeding that of their sporophyte counterparts and can therefore occur beyond the ecological and geographical ranges of their species' sporophytes (Farrar 1998, Ebihara et al 2013, Duffy et al 2015.…”

“…Production of a new sporophyte directly from a gametophyte without syngamy (termed agamospory, a type of apomixis; see figure 1) has arisen independently in a diversity of fern families (Liu et al 2012). Estimates of the proportion of fern species using this reproductive pathway range from 3% (Liu et al 2012) to about 10% (Walker 1984). Although apomictic ferns occur in a variety of habitats, they are especially common in seasonally dry environments (Beck et al 2011, Grusz et al 2014.…”

Living together and living apart: the sexual lives of bryophytes

“…Many ferns possess apogamous life cycles in which gametophytes develop from unreduced spores (diplospory) then bud sporophytes directly from these gametophytes (apogamy) [102]. Such life histories are unknown in bryophytes, although apogamy can be induced experimentally [103].…”

A current perspective on apomixis in ferns

“…This form of asexual reproduction (by spore or seed) is more common in ferns than in any other group of vascular plants. Nearly 10% of the ferns for which reproductive mode has been determined exhibit apomixis (Walker, ; Liu et al, ), compared to less than 1% in flowering plants (Bicknell & Koltunow, ; Becks & Alavi, ). Given that data on reproductive mode are lacking for the vast majority of ferns, this relative contribution of apomicts to fern diversity may well be an underestimate (Liu et al, ).…”

“…Nearly 10% of the ferns for which reproductive mode has been determined exhibit apomixis (Walker, ; Liu et al, ), compared to less than 1% in flowering plants (Bicknell & Koltunow, ; Becks & Alavi, ). Given that data on reproductive mode are lacking for the vast majority of ferns, this relative contribution of apomicts to fern diversity may well be an underestimate (Liu et al, ). The elevated frequency and distinctive meiotic characteristics of apomixis in ferns, which are elaborated upon below, suggest that this form of asexuality may play a more dynamic role in fern evolution than previously considered.…”