Jane F. Sampson

The mating system and population genetic structure in a bird-pollinated mallee, Eucalyptus rhodantha

The mating system and spatial genetic structure of the rare and endangered bird-pollinated mallee Eucalyptus rhodantha were investigated in a remnant stand, using progeny arrays and pollen assayed at four polymorphic allozyme loci. Comparisons of the genetic diversity within and between the pollen pools and maternal parents of two arbitrary subpopulations indicated the presence of spatial genetic heterogeneity which was not broken down by pollen flow. It was suggested that this is the result of a high level if inbreeding and limited pollen dispersal by birds. Estimates of outcrossing rate ranged between [tcirc ]=0·59 and [tcirc ]=0·67 and were at the low end of the range reported for other eucalypts. It was concluded that E. rhodantha has a mixed mating system with a significant proportion of self-pollination. Biparental inbreeding within small neighbourhoods probably also contributed to the high level of inbreeding. The low level of outcrossing observed in E. rhodantha was not consistent with the hypothesis that bird pollination leads to high levels of outcrossing in the Australian flora. However, the level of outcrossing achieved through bird pollination together with high levels of gene flow between populations contribute to the maintenance of the relatively high levels of diversity which characterise the dissected populations of this species.

Outcrossing between an agroforestry plantation and remnant native populations of Eucalyptus loxophleba

Gene dispersal among populations of a species is an important force influencing their genetic structure. Dispersal may also occur between taxa that would normally be isolated when nonendemic, domesticated or transgenic species are planted within the natural range of interfertile taxa. Such a mosaic of populations is typical of many agricultural landscapes, and investigations are needed to assess the risks of genetic contamination of the endemic populations but a combination of approaches may be necessary because of the limitations of research in this landscape. This study used microsatellite markers and a range of analyses (mating system, paternity exclusion, Bayesian assignment) to examine gene dispersal between remnants of the endemic Eucalyptus loxophleba ssp. supralaevis and a plantation of a nonendemic subspecies. Our results indicate that remnant populations are connected by significant dispersal to pollen sources up to 1.94 km away including the plantation. The combined analyses showed that the pollen pool and outcrossing rates of individuals within remnants varied significantly probably because of asynchronous flowering and that the likelihood of paternity was not correlated with spatial proximity. More than half of all progeny had male parents from outside their stand with the largest proportions estimated to come from the plantation by exclusion (42.4%) or Bayesian analyses (18.8–76%). Fragmentation may not be associated with decreased gene dispersal between populations of tree species, natural or planted, so that the distances required to buffer endemic trees in fragmented rural landscapes are likely to be large.

Multiple paternity in Eucalyptus rameliana (Myrtaceae)

Estimates of the level of multiple paternity/correlated outcrossing within and between fruits in a predominantly outbreeding population of the bird-pollinated mallee, Eucalyptus rameliana, were made using six allozyme loci. The correlation of outcrossed paternity (rp) was positive and significant within fruits (0.26) and the effective number of mates for a single fruit was estimated to be 3.85. This correlation was attributed to the low number of potential male parents and pollen-bearing flowers available at any one time in the population. Although consistent with suggestions that correlation of paternity may be a general feature of animal-pollinated plants, the level in E. rameliana was remarkably low considering its population size and phenology. There was no significant correlation of paternity between fruits, probably because flowers were pollinated at different times during the long season. Individual plants differed greatly in both male and female contributions to the next generation, with a small proportion of genotypes located in a small area contributing at least half of the gene pool of the seeds stored in the canopy. In contrast, the male contribution is probably from a wider area. The specialization of floral structure and phenology in E. rameliana for bird-pollination has probably contributed to correlation of paternity within fruits because there are fewer male parents available at any one time than in mass-flowering species. However, the behaviour of the bird pollinators also promotes pollen dispersal and multiplicity of paternity, factors which would be particularly significant when the female contribution to the gene pool is concentrated in a few plants.

Contemporary pollen-mediated gene immigration reflects the historical isolation of a rare, animal-pollinated shrub in a fragmented landscape.

Fragmentation is generally considered to have negative impacts on widespread outbreeders but impacts on gene flow and diversity in patchy, naturally rare, self-compatible plant species remain unclear. We investigated diversity, gene flow and contemporary pollen-mediated gene immigration in the rare, narrowly distributed endemic shrub Calothamnus quadrifidus ssp. teretifolius. This taxon occurs in an internationally recognized biodiversity hotspot subjected to recent human-induced fragmentation and the condition of the remnants ranges from intact to highly degraded. Using microsatellites, we found that inbreeding, historically low gene flow and significant population differentiation have characterized the genetic system of C. quadrifidus ssp. teretifolius. Inbreeding arises from self-pollination, a small amount of biparental inbreeding and significant correlation of outcross paternity but fecundity was high suggesting populations might have purged their lethals. Paternity analyses show that pollinators can move pollen over degraded and intact habitat but populations in both intact and degraded remnants had few pollen parents per seed parent and low pollen immigration. Genetic diversity did not differ significantly between intact and degraded remnants but there were signs of genetic bottlenecks and reduced diversity in some degraded remnants. Overall, our study suggests human-induced fragmentation has not significantly changed the mating system, or pollen immigration to, remnant populations and therefore genetic connectivity need not be the highest conservation priority. Rather, for rare species adapted to higher levels of inbreeding, conservation efforts may be best directed to managing intact habitats and ecosystem processes.

Temporal variation in allele frequencies in the pollen pool of Eucalyptus rhodantha

Investigation of the mallee Eucalyptus rhodantha which has an irregular, protracted flowering period demonstrated that individual plants differ in fecundity and phenology and that allele frequencies in the pollen pool change during the flowering season. Allele frequencies of the pollen pool were estimated by several methods which either assumed temporal homogeneity or took flowering phenology into account. These allele frequencies were compared for their abilities to estimate [tcirc ]. The estimates of [tcirc ] based on allele frequencies which took phenology into account were not consistently higher or better than those that did not at any of the four loci surveyed. Changes in the allele frequencies of the pollen pool during the flowering season may contribute to variation between some single-locus estimates of [tcirc ] in E. rhodantha which assume temporal homogeneity, but it was concluded that they are not the major cause of bias.

Complex hybridity in Isotoma petraea . VIII. Variation for seed aborting lethal genes in the O6 Pigeon Rock population

The highly inbreeding Pigeon Rock population of Isotoma petraea consists of about one-third primitive structural homozygotes and two-thirds derived ring-of-six (or ring-of-ten) complex heterozygotes. A majority of the structural homozygotes exhibit significant proportions of seed abortion in their selfed capsules, ranging from about 1 to 33.8 per cent, while all the structural heterozygotes exhibit significant levels of seed abortion, ranging from 8.8 to 59.9 per cent. Seed abortion ratios appear to be determined by genie interactions which are sensitive to plant growth conditions. It is suggested that seed aborting recessive lethal genes are of adaptive utility and have accumulated in this population because they prevent the allocation of resources to inbred homozygotes, which cannot contribute to future generations. The seed aborting systems may be modelled in terms of recessive seed-aborting lethal genes and independently assorting dominant modifiers of those genes. Appropriate mutations may be generated by the transposition of internal chromosome segments.

Complex hybridity in Isotoma petraea. VII, Assembly of the genetic system in the O6 pigeon rock population

The Pigeon Rock population of Isotoma petraea is highly inbreeding and it is polymorphic with respect to interchange hybridity, allozyme variation and lethal system components. Two thirds of the plants in this population are O6 interchange heterozygotes and may be considered as N.S complex heterozygotes. The N complexes are two interchanges removed from the standard “axillaris” chromosome end sequence characteristic of Isotoma structural homozygotes and they carry the PGM 3 allele. Most of the N complexes are non-transmissible through the pollen and where not so, carry a recessive lethal factor. The associated S complexes in the O6 interchange heterozygotes carry the standard chromosome end sequence and either PGM 1 or PGM 2. It is suggested that in highly inbreeding species with terminally localized chiasmata the occurrence of balanced deleterious recessives within supergenes results in permanent hybridity and that the yield of fully heterozygous offspring must be very low, following selling, or very occasional, following rare cross pollination events. Deterministic models incorporating the above observations and deductions indicate that O6 hybrids should occur at a frequency of approximately 66 per cent under a regime involving approximately 98 per cent recurrent selfing irrespective of whether the N complex is transmissible or not transmissible in the pollen. These values are comparable to those actually found in the Pigeon Rock population. It is concluded that complex hybridity in I. petraea had its origin in the Pigeon Rock population, and arose within highly inbreeding lineages in which mutations contributing to an increasingly efficient transmission of permanently heterozygous parental genotypes were selected.

Microsatellite markers isolated from a polyploid saltbush, Atriplex nummularia Lindl. (Chenopodiaceae)

Atriplex nummularia is a polyploid Australian saltbush which has been identified as a suitable species for use in the rehabilitation of agricultural land affected by salinity. We isolated 12 polymorphic loci for a preliminary assessment of genetic variability and structure within the species as a basis for a breeding programme. Preliminary screening of loci in 40 individuals from two populations revealed multibanded genotypes consisting of up to seven alleles in a single individual, with up to 29 alleles observed at a single locus. The multibanded patterns are consistent with the polyploid status of this species.

A rare, new species of Atriplex (Chenopodiaceae) comprising two genetically distinct but morphologically cryptic populations in arid Western Australia: implications for taxonomy and conservation

Abstract. A rare, new tetraploid Atriplex, restricted to two populations ∼30 km apart in arid Western Australia, is supported as a distinct species by morphological and molecular evidence. Genetic analyses using amplified fragment length polymorphisms (AFLPs) showed significant genetic divergence between the two populations. In contrast, an ordination based on elliptic Fourier descriptors for leaf and bracteole shape did not identify any consistent morphological differentiation. Although the level of genetic differentiation is similar to that previously reported between subspecies in other Atriplex, the populations of the new taxon are described herein as A. yeelirrie K.A.Sheph. & K.R.Thiele, without taxonomic recognition at the intraspecific level. We outline our reasoning for this decision and discuss the implications for appropriate conservation management of the species, structured into two genetically distinct populations.

Confirming the genetic affinity of the ‘Eyres Green’ saltbush cultivar as oldman saltbush (Atriplex nummularia Lindl.)

Until recently, ‘Eyres Green’ was the only commercial cultivar of oldman saltbush available for forage planting and landscape rehabilitation. ‘Eyres Green’ is believed to be a clonal cultivar of the ecologically and economically important octoploid dioecious species Atriplex nummularia (Lindl.), but its actual identity is unknown. The genetic relationship of the ‘Eyres Green’ clonal cultivar to the two subspecies A. nummularia ssp. nummularia and A. nummularia ssp. spathulata (Aellen) was assessed using nuclear microsatellite markers, principal coordinate analysis and Bayesian clustering. ‘Eyres Green’ clustered with ssp. nummularia in all analyses, suggesting that the cultivar was derived from an individual of this subspecies, probably from the north-eastern part of the subspecies’ distribution in eastern Australia. Atriplex species are dioecous and substantially wind-pollinated. Plants of the ‘Eyres Green’ cultivar are female, so risk of genetic contamination from initial plantings is low, but could occur if plantings within pollination distance of natural populations set seed and establish male plants that subsequently become a source of pollen contamination.