The host bias of three epiphytic Aeridinae orchid species is reflected, but not explained, by mycorrhizal fungal associations.

PREMISE OF THE STUDY: The three co-occurring epiphytic orchid species, Sarcochilus hillii, Plectorrhiza tridentata, and Sarcochilus parviflorus vary in host specificity; all are found predominantly on the tree Backhousia myrtifolia but some also associate with a broad range of species. Despite this specialization, no fitness advantage has been detected for adult orchid plants growing on the preferred host. Therefore, we predicted that the host specialization of these orchid species is a consequence of a bias toward particular orchid mycorrhizal fungi, which are in turn biased toward particular woody plant species. METHODS: To test this hypothesis, we sampled representatives of each orchid species on B. myrtifolia and other host species across sites. Rhizoctonia-like fungi were isolated from orchid roots and identified using molecular markers. KEY RESULTS: Three groups of fungi were identified, and the orchid species varied in their specificity for these. All fungal groups were found on the host B. myrtifolia; yet at all sites, only one orchid species, S. hillii, associated with all three groups. CONCLUSIONS: Our results demonstrate that these orchid species did vary in their mycorrhizal specificity; however, the distribution of their mycorrhizal associates did not directly explain their host associations. Rather, we propose that the mycorrhizal relationship of these orchid species is complex and have suggested future avenues of research.

The Influence of Cone Age and Urbanisation on the Diversity and Community Composition of Culturable Seed Fungal Endophytes within Native Australian Banksia ericifolia L.f. subsp. ericifolia.

Seed fungal endophytes play a crucial role in assisting the overall health and success of their host plant; however, little is known about the factors that influence the diversity and composition of these endophytes, particularly with respect to how they change over time and within urban environments. Using culturing techniques, morphological analyses, and Sanger sequencing, we identified the culturable seed fungal endophytes of Banksia ericifolia at two urban and two natural sites in Sydney, New South Wales, Australia. A total of 27 Operational Taxonomic Units were obtained from 1200 seeds. Older cones were found to contain, on average, more colonised endophytes than younger cones. Species richness was also significantly influenced by cone age, with older cones being more speciose. Between urban and natural sites, the overall community composition did not change, although species richness and diversity were greatest at urban sites. Understanding how these endophytes vary in time and space may help provide an insight into the transmission pathways used and the potential role they play within the development and survival of the seed. This knowledge may also be crucial for restoration purposes, especially regarding the need to consider endophyte viability in ex situ seed collection and storage in seed-banking practices.

Capturing Genetic Diversity in Seed Collections: An Empirical Study of Two Congeners with Contrasting Mating Systems.

Plant mating systems shape patterns of genetic diversity and impact the long-term success of populations. As such, they are relevant to the design of seed collections aiming to maximise genetic diversity (e.g., germplasm conservation, ecological restoration). However, for most species, little is known empirically about how variation in mating systems and genetic diversity is distributed. We investigated the relationship between genetic diversity and mating systems in two functionally similar, co-occurring species of Hakea (Proteaceae), and evaluated the extent to which genetic diversity was captured in seeds. We genotyped hundreds of seedlings and mother plants via DArTseq, and developed novel implementations of two approaches to inferring the mating system from SNP data. A striking contrast in patterns of genetic diversity between H. sericea and H. teretifolia was revealed, consistent with a contrast in their mating systems. While both species had mixed mating systems, H. sericea was found to be habitually selfing, while H. teretifolia more evenly employed both selfing and outcrossing. In both species, seed collection schemes maximised genetic diversity by increasing the number of maternal lines and sites sampled, but twice as many sites were needed for the selfing species to capture equivalent levels of genetic variation at a regional scale.

Positive selection in AvrP4 avirulence gene homologues across the genus Melampsora.

Pathogen genes involved in interactions with their plant hosts are expected to evolve under positive Darwinian selection or balancing selection. In this study a single copy avirulence gene, AvrP4, in the plant pathogen Melampsora lini, was used to investigate the evolution of such a gene across species. Partial translation elongation factor 1-alpha sequences were obtained to establish phylogenetic relationships among the Melampsora species. We amplified AvrP4 homologues from species pathogenic on hosts from different plant families and orders, across the inferred phylogeny. Translations of the AvrP4 sequences revealed a predicted signal peptide and towards the C-terminus of the protein, six identically spaced cysteines were identified in all sequences. Maximum likelihood analysis of synonymous versus non-synonymous substitution rates indicated that positive selection played a role in the evolution of the gene during the diversification of the genus. Fourteen codons under significant positive selection reside in the C-terminal 28 amino acid region, suggesting that this region interacts with host molecules in most sequenced accessions. Selection pressures on the gene may be either due to the pathogenicity or avirulence function of the gene or both.

Coevolution with higher taxonomic host groups within the Puccinia/Uromyces rust lineage obscured by host jumps.

Partial beta-tubulin 1 sequence data were obtained for 80 taxa of Pucciniaceae, with hosts from 33 angiosperm families, covering all major ordinal groups in the Angiosperm Phylogeny Group classification. As in previous studies, most species of Puccinia and Uromyces fell into two main clades (I and II), with P. glechomatis and P. psidii excluded from Pucciniaceae. Results suggest two processes; a coevolution of and hosts in each clade, as well as associated frequent jumps to ecologically close, but taxonomically distant, hosts. Clade I contained all rusts on Cyperaceae and Juncaceae, and most rusts on host orders from rosids to euasterids II. Clade II contained all rusts on Poaceae, and most on host orders from monocots to core eudicots. In both main clades, several well-supported subclades were identified. The grouping in clade I, subclade E of rusts of Cyperaceae and Asteraceae and, in particular, of an Australian isolate of P. dioicae with rusts on Australian families of Asterales, suggested a local radiation, and supported the coevolutionary relationship between rusts on these two families seen with a different range of asteraceous rusts in the Northern Hemisphere. In clade I, two clades contained only rusts of Asteraceae and Fabaceae, respectively, and in clade II, subclade F contained only rusts of pooid hosts. Rusts on non-pooid hosts were separated from them in subclade G. Other subclades contained a range of rusts on distantly related angiosperm families. Urediniospore morphology was often, but not always, correlated with the molecular phylogeny. Most rusts with urediniospores having few (1-5) equatorial germ pores were in clade I, whereas most with spores having several (5-14) scattered pores were in clade II. The distribution of telial host families on the beta-tubulin rust phylogeny was not random. Aecial hosts of heteroecious rusts played an important role in the evolutionary process. Possible examples of host jumps were seen in rusts on Geraniaceae, Polygonaceae, and Apiaceae. Despite such jumps obscuring past host associations, possible ancestral hosts were identified by the pattern of host distribution at higher taxonomic levels along the ss-tubulin phylogeny. Results suggest that clade I diverged with Cyperaceae, Juncaceae, and the more advanced core eudicot orders (rosids and asterids), whereas clade II diversified with earlier angiosperm groups, such as monocots, Poaceae, and Ranunculales. Qualified support was given to the hypothesis that rusts can reveal taxonomic relationships between their hosts, at genus, family, and ordinal levels.