Relative frequency of sympatric species influences rates of interspecific hybridization, seed production and seedling performance in the uncommon Eucalyptus aggregata

Abstract

Summary 1.Habitat fragmentation can alter the relative frequency of cross-compatible species within an area, which can affect the levels of interspecific hybrid production and reduce the viability of small populations through genetic and demographic swamping. For 18 populations of Eucalyptus aggregata , we examined the effects of absolute and relative population size (compared with its congeners E. rubida , E. viminalis and E. dalrympleana ) on hybrid production, genetic diversity and subsequent seed production and seedling performance. 2. Relative population size was strongly negatively correlated with rates of hybrid seed production, suggesting increased hybridization when the potential sources of interspecific pollen outnumber the sources of intraspecific pollen for E. aggregata trees. 3. Genetic diversity was negatively correlated with relative population size which suggests that hybridization may maintain diversity lost through bottlenecks and drift following reductions in population size. However, the presence of fertile hybrid adults, and introgressed leaf traits within populations exhibiting high hybridization rates, suggests that small E. aggregata populations may be vulnerable to genetic swamping by common congeners. 4. Amongst an array of population parameters (population sizes, genetic diversity and inbreeding), seed production was only positively correlated with relative population size, whereby sites with low relative population sizes tended to produce fewer seed. This could be due to the action of pre-zygotic barriers which removes inviable hybrid genotypes as levels of interspecific pollen flow increase. 5. Germination and survivorship displayed a similar positive correlation with relative population size, suggesting post-zygotic hybrid breakdown may also contribute towards to demographic swamping of remnant populations. 6. Synthesis . Our results suggest that relative population size is an important parameter determining rates of hybrid production, seed production and seedling performance. Furthermore, relative population size has stronger effects on population fecundity than absolute population size, genetic diversity and levels of inbreeding. Relative population sizes > 0.5 (i.e. at least equal frequencies of parentals) may be required to avoid the deleterious effects of genetic and demographic swamping on the viability of rare species.

Clonality, interspecific hybridisation and inbreeding in a rare mallee eucalypt, Eucalyptus absita (Myrtaceae), and implications for conservation

“…Interspecific hybridisation is common in eucalypts at lower taxonomic levels, and similarly high rates in seed cohorts have also been reported in another rare clonal mallee, E. argutifolia (up to 47 % of seeds) (Kennington and James 1997b) and in populations of E. benthamii (up to 30 %) (Butcher et al 2005) and E. aggregata (up to 30 %) (Field et al 2008(Field et al , 2009. Hybridisation may be facilitated in E. absita by pollen pool swamping, which can increase the rates of hybridisation in rare eucalypts due to an abundance of interspecific relative to intraspecific outcrossed pollen (Field et al 2008).…”

“…While a longterm, natural phenomenon in eucalypts, the rate of interspecific hybridisation can substantially increase in the seed progeny of remnant and rare species when the availability of intraspecific pollen is less abundant than that of common, co-occurring congeners (e.g. E. aggregata, Field et al 2008). Inbreeding in the form of self-fertilisation (selfing) or mating between close relatives (biparental inbreeding), is known to result in inbreeding depression in eucalypts, impairing fitness and compromising genetic quality of seed collections, as it can result in both short-and long-term detriments to growth and survival (Eldridge and Griffin 1983;Potts and Wiltshire 1997;Costa e Silva et al 2011).…”

Reproductive isolation and the expansion of an invasive hybrid swarm

“…It is also possible that interspecific competition is promoting hybridization since aggression can dictate the outcome of malemale competition for spawning territory (KodricBrown and Rosenfield 2004). Greater fecundity in red shiner females compared to blacktail shiner females could also promote hybridization because factors that alter the relative frequency of cross-compatible species can affect levels of hybrid production (Field et al 2008).…”

Prezygotic barriers to gene flow between Taraxacum ceratophorum and the invasive Taraxacum officinale (Asteraceae)

“…Pollinators making heterospeciWc visits from T. oYcinale inXorescences to T. ceratophorum could deposit either pure or mixed pollen loads. As T. oYcinale pollen increases in frequency over the course of local invasion, it is more likely that stigmas of some native Xorets will receive only heterospeciWc pollen in addition to the autonomous and geitonogamous movement of self-pollen within an inXorescence (Field et al 2008;Levin 1975;Levin et al 1996;Prentis et al 2007). Under these circumstances, the "leaky" combination of ploidy and mentor barriers should determine the rate of hybridization.…”