A densely sampled, dated phylogeny of the Australian Rhamnaceae tribe Pomaderreae (~240 woody shrub species, 90% sampled) shows polyploidy in about 44% of species and 7 of 10 genera, a significant positive association between polyploidy and genus-level species richness, but no significant association with tip diversification rates (diversification rates estimated per extant species at the tips of the phylogeny) and no broader niche-space occupancy. The source study (Nge et al., Annals of Botany 135(3): 531-548) explicitly leaves open whether ancient whole-genome duplication (WGD) events occurred in Pomaderreae and the wider Rhamnaceae, and when, and which mechanisms explain the observed uncoupling of polyploidy from diversification rates. The problem asks for transcriptome-scale detection and dating of ancient WGD events across the tribe - in particular at the crowns of the species-rich genera Pomaderris, Spyridium and Cryptandra - together with macroevolutionary tests that adjudicate among candidate explanations for the richness-rate uncoupling: lag time between polyploidization and diversification, elevated extinction of polyploid lineages invisible to extant-only phylogenies, independent genome-size effects, and trait-dependent diversification. The same framework must account for two concrete instances of the pattern: polyploid Pomaderris lineages that dispersed repeatedly to New Zealand yet failed to radiate there, and the scarcity and non-radiation of Pomaderreae lineages that entered the Australian arid biome.

Background

Rhamnaceae, the buckthorn family, is a cosmopolitan flowering-plant family whose Australasian tribe Pomaderreae comprises about 240 species of woody shrubs and small trees in ten genera, the largest being Pomaderris (~70 species). Polyploidy is the condition of possessing more than two complete chromosome sets, arising through whole-genome duplication (WGD); polyploids derived from recent events (neopolyploids) contrast with paleopolyploids, in which ancient WGD has been followed by gene loss and chromosomal rearrangement ('re-diploidization') so that the duplication is detectable only statistically. Odd ploidies such as triploidy (three sets) usually reduce fertility and cause chromosomal instability, so persistent high triploid frequency is itself evolutionarily noteworthy. Polyploidy has repeatedly been linked to plant species richness and diversification - the speed at which lineages add species - but the direction and generality of the link are contested: analyses of young polyploids have found them diversifying at lower rates and behaving as 'evolutionary dead-ends' (Mayrose et al. 2011, Science 333: 1257), whereas a family-level survey of angiosperms found WGD events frequently followed by diversification-rate increases (Landis et al. 2018), and global geography shows polyploid frequency varying strongly with environment, being lower in stable regions (Rice et al. 2019). One widely hypothesized mechanism is that duplicated genomes widen ecological tolerance, letting polyploids occupy broader or more extreme niches than diploid relatives.

For Pomaderreae specifically, earlier flow-cytometry work on the genus Pomaderris found widespread polyploidy (about 46% of sampled species, mostly triploid) together with apomixis - asexual seed production - with conservation consequences (Chen et al. 2019), and a dated molecular phylogeny showed continental vicariance in Australia plus repeated independent dispersals of Pomaderris to New Zealand in the Pliocene-Pleistocene (Nge et al. 2021). A family-wide phylogenomic analysis of Rhamnaceae found that high temperate-zone diversity reflects rapid in-situ diversification from the Oligocene onward, but did not test ploidy or WGD timing (Tian et al. 2024). Nge et al. (2025) then generated a tribe-wide, dated phylogeny (215/240 species) from 30 single-copy nuclear loci, estimated the ploidy of each sequenced species with nQuire - a statistical method that infers ploidy from the ratio of allele frequencies in sequencing reads - and obtained tip diversification rates (per-extant-species diversification rates) with MiSSE, a phylogenetic birth-death model, also modelling species' climate niches from distribution records. Their results define the puzzle this problem addresses: extensive polyploidy (44.1% of 238 species; 70% of genera; an unusually high 26.4% triploids), a significant positive relationship between polyploidy and genus size (species richness), but only a non-significant positive relationship with tip diversification rates, no wider niche occupancy for polyploids (which in fact occupy wetter niches in eastern Australia), few transitions into the arid biome, and monotypic south-western Australian genera that are all diploid. The authors point out that extinction-rate estimates from extant-only phylogenies are notoriously unreliable, so an elevated extinction of polyploid lineages could be hidden from their analyses. Detecting and dating the ancient WGD events themselves would require transcriptome-scale data, which do not yet exist for the tribe.

Problem Statement

For the Australian Rhamnaceae tribe Pomaderreae, determine whether ancient whole-genome duplication (WGD) events occurred in the tribe's evolutionary history and when, and whether they explain the pattern established by Nge et al. (2025): polyploidy is significantly and positively associated with genus-level species richness, yet shows no significant association with tip diversification rates, no broader niche-space occupancy, and no role in arid-biome colonization. Concretely, the problem comprises three jointly necessary components serving this single objective: (1) Detect and date ancient WGD events across Pomaderreae and the wider Rhamnaceae from transcriptome- or genome-scale data, and establish whether WGD events coincide with the crowns of the species-rich genera Pomaderris, Spyridium and Cryptandra, where diversification pulses might have produced the present richness gradient; a well-supported conclusion that no Pomaderreae-specific WGD is detectable is a possible outcome, and it requires the search's power at the relevant time depths to be established. (2) Adjudicate among the candidate mechanisms for the uncoupling of polyploidy from extant-tip diversification: a temporal lag between polyploidization and diversification such that old WGDs drove deep radiations while extant-tip rates are uncoupled; elevated extinction of polyploid lineages that extant-only phylogenies cannot register; independent effects of genome-size (C-value) variation; and interactions between polyploidy and ecological or morphological traits (leaf form, indumentum - the hair covering of leaves - spinescence, ant dispersal of seeds, floral traits), which requires explicitly trait-dependent diversification analysis. (3) Account, within the same explanatory framework, for the two concrete instances of the pattern: the repeated independent dispersals of polyploid Pomaderris lineages to New Zealand in the Pliocene-Pleistocene that nevertheless failed to radiate there, and the few Pomaderreae transitions into the Australian arid biome whose lineages have likewise failed to radiate. The dated WGD history, the mechanisms for the richness-rate uncoupling, and the New Zealand and arid-biome instances must be resolved consistently within a single explanatory framework.

References

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