The effect of small population size on the mating system of a rare clonal mallee, Eucalyptus argutifolia (Myrtaceae)

Abstract

Outcrossing rates, seed yields and the incidence of seed abortion were estimated in differentsized populations of a rare clonal mallee, Eucalyptus argutifolia Grayling and Brooker. Multilocus estimates of the outcrossing rate were high in most populations (tm = 0.79-0.96), and no relationship between population size and the outcrossing rate was evident. In addition, significant amounts of interspecific hybridization were found in some small populations (up to 47 per cent of the seeds assayed). These estimates were much higher than expected, as it was apparent that the potential for geitonogamous pollination far exceeded that of outcrossing. Pollination experiments indicated that E. argutifolia is self-compatible, and therefore the higher than expected outcrossing rates were attributed to inbreeding depression. This view was supported by substantial levels (over 50 per cent) of seed abortion. Selection against homozygotes was also evident during later stages of development, and this resulted in adult populations having genotypic proportions similar to those expected under random mating (complete outcrossing). It was suggested that this selection later in the life cycle offset the purging of early-acting lethal (and semi-lethal) recessives and may explain why even small populations maintain high genetic loads. Similar explanations may be applied to other mass-flowering eucalypts that maintain strong inbreeding depression despite a mixed-mating system.

Altered Soil Properties Inhibit Fruit Set but Increase Progeny Performance for a Foundation Tree in a Highly Fragmented Landscape

“…We found no obvious relationship between effective outcrossing rates estimated in seed progeny and population fragmentation variables, consistent with many other studies in eucalypts (e.g., Ottewell et al, 2009;Breed et al, 2012Breed et al, , 2015. This is most likely due to the post-zygotic seed selection mechanisms in eucalypts that preferentially eliminate selfed progeny, with the surviving seeds that are used to estimate mating system parameters largely the result of outcrossing (see Kennington and James, 1997).…”

“…The most likely explanation for the observed pattern is that trees in small populations receive lower quality pollen through higher proportional transfer of self-pollen. Although we found no decline in effective outcrossing rate, the likelihood of increased selfing in small populations is consistent with evidence from other Eucalyptus species that incomplete pre-or post-zygotic ovule abortion mechanisms typically lead to reduced seed set when faced with increased self-pollination (Griffin et al, 1987;Ellis and Sedgley, 1992;Kennington and James, 1997;Pound et al, 2003;Krauss et al, 2007;Gauli et al, 2014). In addition, the very high rates of pollen immigration into small E. wandoo populations ) appear insufficient to mitigate the impact of high selfing rates on seed set.…”

Edge effects enhance selfing and seed harvesting efforts in the insect-pollinated Neotropical tree Copaifera langsdorffii (Fabaceae)

“…Thus, the higher levels of selfing may explain the low observed heterozygosity at the edge. However, in general, selfing in tropical tree species produces inbreeding depression, causing the mortality of selfed seeds and individuals as was observed in Eucalyptus argutifolia (Kennington and James, 1997), Eucalyptus marginata (Millar et al, 2000), Pseudobombax munguba (Gribel and Gibbs, 2002), Platypodium elegans (Hufford and Hamrick, 2003) and Neobalanocarpus heimii (Naito et al, 2005). We observed significant higher germination in the SW than the edge, suggesting that inbreeding depression was higher in seeds from the edge, where selfing was highest.…”

Mating system and multiple paternity in Hedysarum coronarium L. (Fabaceae)

“…This view is supported by high levels of seed abortion in hand self-pollination when compared to open-pollination. Thus, higher levels of embryo abortion may be responsible for maintaining elevated levels of outcrossing rates in populations [21]. The protandrous nature of flower development in H. coronarium, sequential flowering on the same inflorescence during anthesis period and competitive inferiority of self pollen reported for this species [8] are additional causes of a high outcrossing rate.…”

“…In the Tunis population, t m for maternal plants were more dispersed and hence may explain a low correlation of outcrossing within progeny arrays (r t = 0.053). Variation in estimates of single-plant outcrossing rates may arise from variation in the distance to nearest neighbours [36] (as in the El Alia population), genetic differences in levels of self-fertility [6] and asynchrony in flowering times [13,21,24].…”