Crepis

As with several other genera in Asteraceae, multiple Crepis species exhibit apomixis, a form of asexual reproduction where flowers produce clonal seeds without need of fertilization.[8] Unlike sexually-reproducing individuals, which are diploid, apomicts are typically polyploid, with three or more sets of chromosomes.[9] The mechanism of apomixis in Crepis is apospory, wherein diploid tissue arises during meiosis in the ovule. Through apospory, asexual seeds can develop spontaneously in the flowers of a non-apomictic Crepis.[10]

Crepis is part of the Cichorieae tribe in Asteraceae, and is one of several genera of that tribe that exhibits apomixis.[11][12][13] The largest group of asexual species is found in North America, and is referred to as the "North American Crepis agamic complex"[8][9] The agamic complex may have first arisen in the Pliocene, between 5.3 and 2.6 million years ago.[14] At least one Eurasian Crepis species, C. tectorum, has been observed to self-fertilize.[6] Another agamic complex is thought to exist in Asia.[15] Species known to produce apomicts include C. acuminata, C. barbigera, C. intermedia, and C. occidentalis. Polyploid individuals may still be able to carry out sexual reproduction and therefore produce sexual offspring, allowing for the formation of new polyploid hybrids.[6]

Apomictic individuals can prevent reproduction in sexual individuals in multiple ways. Like other apomictic species in Cichorieae, polyploid apomicts may still produce viable pollen that can be transferred to sexual diploids through the normal process of pollination.[16] The mixed apomict-sexual parantage may produce a polyploid hybrid (as has been observed when the apomictic C. barbigera breeds with diploid species C. atribarba[17]). Seed set in the diploid may be reduced, or the pollen may produce hybrid seeds that are unable to reproduce or survive to a normal lifespan.[18] The pollen from the apomict may also prevent germination of pollen from a diploid flower. When pollen from a polyploid individual fertilizes diploid flowers, the resultant seeds are typically polyploid, which lowers the diploid population over generations.[9]

As a result of this reproductive interference, sexual reproduction between diploid plants is reduced when apomicts enter a population. Because diploid species require pollen exchange to reproduce and apomicts can produce seeds by parthenogenesis, apomicts do not suffer any reproductive consequences in a mixed diploid-polyploid population. Meanwhile, the reproductive success of sexually-reproducing diploids falls, resulting in diploids being found more often in populations isolated from apomicts.[9]

Although apomixis has been considered a "blind alley of evolution", research into apomictic species and species complexes has cast doubt on this. Apomictic clades elsewhere in Cichorieae have demonstrated an ability to "revert" to a sexual mode of reproduction, reducing the risk of extinction through lack of introgression.[8] Speciation in Crepis has occurred through its ability to self-fertilize, hybridize, and form polyploid apomicts.[6]

Apomixis can facilitate range expansion in ways that sexual reproduction does not. Apomicts do not require input of genetic material for another individual, and can therefore produce seeds on maturity without the aid of pollinators. This allows apomictic populations to expand into new geographic areas more rapidly than sexual ones. Because apomixis in Crepis involves multiple ploidy levels, there is sufficient genetic variation for adaptation to novel ecosystems.[19] Additionally, some Crepis apomict species have the ability to hybridize with other apomicts, resulting in genetic recombination alongside parthenogenesis.[20]