Investigating historical drivers of latitudinal gradients in polyploid plant biogeography: A multi-clade perspective

Premise of the Study

The proportion of polyploid plants in a community increases with latitude, and different hypotheses have been proposed about which factors drive this pattern. Here, we aim to understand the historical causes of the latitudinal polyploidy gradient using a combination of ancestral state reconstruction methods. Specifically, we assess whether (1) polyploidization enables movement to higher latitudes (i.e., polyploidization precedes occurrences in higher latitudes) or (2) higher latitudes facilitate polyploidization (i.e., occurrence in higher latitudes precedes polyploidization).

Methods

We reconstruct the ploidy states and ancestral niches of 1,032 angiosperm species at four paleoclimatic time slices ranging from 3.3 million years ago to the present, comprising taxa from four well-represented clades: Onagraceae, Primulaceae, Solanum (Solanaceae), and Pooideae (Poaceae). We use ancestral niche reconstruction models alongside a customized discrete character evolution model to allow reconstruction of states at specific time slices. Patterns of latitudinal movement are reconstructed and compared in relation to inferred ploidy shifts.

Key Results

We find that no single hypothesis applies equally well across all analyzed clades. While significant differences in median latitudinal occurrence were detected in the largest clade, Poaceae, no significant differences were detected in latitudinal movement in any clade.

Conclusions

Our preliminary study is the first to attempt to connect ploidy changes to continuous latitudinal movement, but we cannot favor one hypothesis over another. Given that patterns seem to be clade-specific, a larger number of clades must be analyzed in future studies for generalities to be drawn.

Methods
Phylogenies were collected from the following sources: Onagraceae (Freyman and Höhna 2019), Primulaceae (De Vos et al. 2014), Solanaceae (Särkinen et al. 2013), and Poaceae (Spriggs et al. 2014). The phylogenies of Solanaceae and Poaceae were pruned to retain just those species in Solanum and Pooideae respectively. Occurrence points for species in each clade were downloaded from GBIF, and were cleaned for inaccuracies following protocols similar to those in Boyko et al. (2023). Ploidy data were gathered from the supplementary data of Rice et al. (2019).

The ancestral ranges of taxa in each clade were reconstructed using the program machuruku (Guillory and Brown 2021) to time periods available from PaleoClim data (Brown et al. 2018): the Last Interglacial (LIG, c. 130 ka), Marine Isotope Stage 19 (MIS19, c. 787 ka), the mid-Pliocene Warm Period (mPWP, c. 3.205 Ma), and Marine Isotope Stage M2 (M2, c. 3.3 Ma). Ploidy was reconstructed using corHMM (Beaulieu et al. 2013; Boyko and Beaulieu 2021) with modified functions that allow for ancestral state reconstruction at specific time slices rather than at nodes.

Finally, the data gathered from these analyses were examined using a variety of statistical techniques.