Paternity analysis reveals wide pollen dispersal and high multiple paternity in a small isolated population of the bird-pollinated Eucalyptus caesia (Myrtaceae)

Abstract

Optimal foraging behaviour by nectavores is expected to result in a leptokurtic pollen dispersal distribution and predominantly near-neighbour mating. However, complex social interactions among nectarivorous birds may result in different mating patterns to those typically observed in insect-pollinated plants. Mating system, realised pollen dispersal and spatial genetic structure were examined in the bird-pollinated Eucalyptus caesia, a species characterised by small, geographically disjunct populations. Nine microsatellite markers were used to genotype an entire adult stand and 181 seeds from 28 capsules collected from 6 trees. Mating system analysis using MLTR revealed moderate to high outcrossing (t_m=0.479-0.806) and low estimates of correlated paternity (r_p=0.136±s.e. 0.048). Paternity analysis revealed high outcrossing rates (mean=0.72) and high multiple paternity, with 64 different sires identified for 181 seeds. There was a significant negative relationship between the frequency of outcross mating and distance between mating pairs. Realised mating events were more frequent than expected with random mating for plants <40 m apart. The overall distribution of pollen dispersal distances was platykurtic. Despite extensive pollen dispersal within the stand, three genetic clusters were detected by STRUCTURE analysis. These genetic clusters were strongly differentiated yet geographically interspersed, hypothesised to be a consequence of rare recruitment events coupled with extreme longevity. We suggest that extensive polyandry and pollen dispersal is a consequence of pollination by highly mobile honeyeaters and may buffer E. caesia against the loss of genetic diversity predicted for small and genetically isolated populations.

Figure 1

Distribution of Eucalyptus caesia subsp. caesia (closed circles) and E. caesia subsp. magna (open circles) in the Central Wheatbelt of Western Australia, and location of sampled plants at the study site in Boyagin Nature Reserve (32°28'21.0"S, 116°52'23.5"E), generated using Google Earth imagery (2014). Arrows indicate trees from which seeds were collected for paternity analysis. Locations of E. caesia populations are based on on-ground survey by SD Hopper and Western Australian Herbarium records.

Figure 2

Eucalyptus caesia subsp. caesia growing on slopes of a granite bornhardt in Boyagin Nature Reserve, Western Australia. Photos shown include a single multi-stemmed plant (top left), a New Holland honeyeater foraging for nectar (bottom left; photo by SD Hopper), E. caesia flowers (bottom right) and multiple stems comprising a single multilocus genotype (top left). Photos are by N Bezemer unless otherwise indicated.

Figure 3

Distribution of outcross pollen donors for six Eucalyptus caesia subsp. caesia located on the summit of a granite bornhardt in Boyagin Nature Reserve, Western Australia (GDA 94, Zone 50). Axis values are UTM easting (x axis) and northing (y axis). Data are based on the results of a paternity assignment (n=39 (Tree 1), 8 (Tree 5), 26 (Tree 10), 14 (Tree 17), 31 (Tree 101) and 12 (Tree 114)) conducted in CERVUS.

Figure 4

Proportion of realised outcross mating events with distance, showing (a) observed and random outcross pollination events with geographic distance (m) between plant pairs and (b) proportion of mating events with ranked distance between plant pairs. Realised mating events are based on the results of a paternity assignment of seeds (n=130) collected from 6 Eucalyptus caesia subsp. caesia in Boyagin Nature Reserve, Western Australia. Potential mating events are the distances between maternal trees and all potential sires. *Significant differences (P<0.01) between observed and potential mating events.

Figure 5

Genetic differentiation in a stand of Eucalyptus caesia subsp. caesia located in Boyagin Nature Reserve, Western Australia (GDA 94, Zone 50H), showing (a) assignment of individuals to genetic clusters based on maximum likelihood analysis in STRUCTURE, (b) geographic location of individuals with ⩾60% inferred membership to one of three clusters and (c) principle coordinates analysis (PCoA) plots of axes 1 vs 2 and 1 vs 3 and the total variation explained.