Ecological and evolutionary opportunities of apomixis: insights from Taraxacum and Chondrilla

Abstract

The ecological and evolutionary opportunities of apomixis in the short and the long term are considered, based on two closely related apomictic genera: Taraxacum (dandelion) and Chondrilla (skeleton weed). In both genera apomicts have a wider geographical distribution than sexuals, illustrating the short-term ecological success of apomixis. Allozymes and DNA markers indicate that apomictic populations are highly polyclonal. In Taraxacum, clonal diversity can be generated by rare hybridization between sexuals and apomicts, the latter acting as pollen donors. Less extensive clonal diversity is generated by mutations within clonal lineages. Clonal diversity may be maintained by frequency-dependent selection, caused by biological interactions (e.g. competitors and pathogens). Some clones are geographically widespread and probably represent phenotypically plastic 'general-purpose genotypes'. The long-term evolutionary success of apomictic clones may be limited by lack of adaptive potential and the accumulation of deleterious mutations. Although apomictic clones may be considered as 'evolutionary dead ends', the genes controlling apomixis can escape from degeneration and extinction via pollen in crosses between sexuals and apomicts. In this way, apomixis genes are transferred to a new genetic background, potentially adaptive and cleansed from linked deleterious mutations. Consequently, apomixis genes can be much older than the clones they are currently contained in. The close phylogenetic relationship between Taraxacum and Chondrilla and the similarity of their apomixis mechanisms suggest that apomixis in these two genera could be of common ancestry.

Genetic diversity and reproductive biology in ecotypes of the facultative apomict Hypericum perforatum L.

“…In this way, facultative apomicts not only follow the strategy adopted by strict selfers but also ensure maintenance of high levels of heterozygosity and polyploidy. Such an hypothesis has been put forward for the two apomictic genera Taraxacum and Chondrilla, as it is thought that gene flow between apomicts and their sexual relatives has enabled apomixis to be maintained for longer evolutionary periods than originally predicted (van Dijk, 2003).…”

Insights into developmental processes in anthers, ovaries, and ovules of Taraxacum belorussicum (Asteraceae-Cichorioideae) using DIC optics

“…Results of these studies, as well as the present observations of the ovule anatomy and female gametophyte structure in T. belorusscium, clearly indicate that, although in apomictic dandelions the development of both embryo and endosperm is autonomous, their ovules and unreduced female gametophytes permanently retain features that are associated with sexual reproduction. This can be explained by the fact that apomictic Taraxacum species are considered to be relatively recent in origin and hence the functionless characters are still maintained (Richards 1973;Maynard Smith 1978;Van Dijk 2003). On the other hand, Płachno et al (2015) pointed out that the presence of vital micropylar transmitting tissue and synergid cells with a filiform apparatus makes no barrier for pollen tube to reach the ovule and female gametophyte, thus fertilization of an unreduced egg cell of apomicts is likely, which may be essential in the light of possible hybridization among sexuals and apomicts and the creation of new apomictic lineages, advantageous from an evolutionary point of view.…”

“…Only autonomous apomicts are entirely independent of pollination for the embryo and endosperm development. Autonomous apomixis is particularly frequent within the Asteraceae and occurs mainly among diplosporous species, like Taraxacum and Chondrilla taxa (Nogler1984; Asker and Jerling 1992;Van Dijk 2003;Kościńska-Pająk and Bednara 2006;Noyes 2007). Although in autonomous apomicts, the endosperm, similar to the embryo, develops autonomously, most of them maintain the male function and continue to produce functional pollen.…”

“…The majority of apomictic plants is pseudogamous (Nogler 1984;Mogie 1992;Koltunow and Grossniklaus 2003;Van Dijk 2003, 2009). They are capable of developing embryos independent of fertilization of the egg cell; however, pollination is necessary for the fertilization of the central cell and the initiation of endosperm development essential for viable seed formation.…”

Male sterility in triploid dandelions: asexual females vs asexual hermaphrodites

“…Ruderalia (Richards, 1986). This pollen production has been seen as a pure waste of resources (Maynard Smith, 1978;Van Dijk, 2003) since asexual dandelions do not require pollination to trigger endosperm development. Furthermore, asexual dandelions are predominantly triploid and, therefore, as a result of unbalanced meiosis, most of the produced pollen is inviable.…”