Technical Comments

Comment on “Eocene Fagaceae from Patagonia and Gondwanan legacy in Asian rainforests”

See allHide authors and affiliations

Science  15 Nov 2019:
Vol. 366, Issue 6467, eaaz2189
DOI: 10.1126/science.aaz2189

Abstract

Wilf et al. (Research Articles, 7 June 2019, eaaw5139) claim that Castanopsis evolved in the Southern Hemisphere from where it spread to its modern distribution in Southeast Asia. However, extensive paleobotanical records of Antarctica and Australia lack evidence of any Fagaceae, and molecular patterns indicate shared biogeographic histories of Castanopsis, Castanea, Lithocarpus, and Quercus subgenus Cerris, making the southern route unlikely.

In a recent paper (1), Wilf et al. described fossils belonging to the north hemispheric family Fagaceae (oak family) from sediments of Patagonia dated to 52.2 million years (Ma) ago, the early Eocene. The fossils comprise one immature (pistillate) infructescence and four mature fruits attached to an axis and are assigned to the extant genus Castanopsis on the basis of a DNA-scaffold analysis using seven scored morphological traits. Wilf et al. state that “the new fossils represent … the oldest record, by ~8 million years, of the genus Castanopsis” and conclude that “Castanopsis evolved in the Southern Hemisphere” and, moving along a “southern route,” provided the stock for the modern survivors of Castanopsis, ~120 to 130 tree species ranging from northwestern India to New Guinea and Japan. They speculate that this ancestral Castanopsis represents one of several paleo-Antarctic plant genera that are today in Southeast Asian rainforests. Consequently, numerous younger fossils from North America and Eurasia previously assigned to Castanopsis must represent “more distant relatives of the extant genus” than the Patagonian fossils.

We acknowledge the importance of the fossil that geographically extends the record of Fagaceae but suggest an alternative evolutionary and biogeographic interpretation that takes into account genetic differentiation patterns of modern genera.

We (i) show that the character suite that links the Patagonian fossil to modern Castanopsis is plesiomorphic, making its generic assignment ambiguous. We (ii) use the extensive pollen and macrofossil record (Australia, Antarctica) to demonstrate that a southern route of Fagaceae to Southeast Asia currently lacks any fossil evidence. Finally, (iii) molecular data reject geographically isolated early evolutionary histories of the castaneoid genera Castanea, Castanopsis, and Lithocarpus and link them to the Eurasian Quercus subgenus Cerris.

Regarding (i), we note that slightly younger infructescences from Tennessee described as Castanopsoidea (2) share features with the fossil from Patagonia but differ by three-flower cupules, a condition also present in modern Castanopsis; Castanopsis rothwellii from Patagonia shows a character suite that is distinctly primitive within the paraphyletic Castaneoideae [Fig. 1 and Table 1; character 7, inflorescence sexuality, was coded as unisexual for extant Castanopsis and Lithocarpus; this should be unisexual and mixed instead (3)]. On the basis of the available data, it is impossible to decide whether Castanopsoidea and Castanopsis rothwellii represent stem Castaneoideae/Fagaceae, are extinct sister lineages of Castanea-Castanopsis, or belong to the modern genus.

Fig. 1 Phylogenetic information content of the seven-character matrix used by Wilf et al. when using parsimony.

Phylogenetic framework is from Oh and Manos (9) with a paraphyletic Castaneoideae. Character 7 was incorrectly coded in Wilf et al. for Castanopsis and Lithocarpus; character 5 was incorrectly coded for Fagus. Character states for characters 1 to 7 are indicated by open or solid squares (top to bottom) for terminal taxa; reconstructed character suites (using the standard parsimony model implemented in Mesquite version 3.6) are shown for selected hypothetical common ancestors. Tree length is 20. Colors of branches indicate numbers of additional steps (evolutionary changes) required by placing the fossil(s) on the corresponding branch. Note that the number of steps does not change whether C. rothwellii is placed as sister to all Fagaceae, sister to Fagus, sister to trigonobalanoids and quercoids, or as part of Castanopsis. Asterisks indicate genera not included in Wilf et al.’s analysis.

Table 1 Emended and corrected seven-character matrix.

Boldface entries are corrected from the Wilf et al. matrix.

View this table:

Regarding (ii), the southern route of Castanopsis to Asia, we note that evidence for such a pathway is currently missing. First, the genus was present in North America in the late early Eocene (4) less than 4 Ma after C. rothwellii. The revised age of the Nut Beds flora in Oregon is 48.32 Ma (5). In addition, the genus was present in Europe during the Eocene. Second, despite extensive paleopalynological and macrofossil work in Antarctica and Australia/Tasmania, dispersed pollen, leaves, or reproductive structures of Castaneoideae or any other Fagaceae, common in the Northern Hemisphere during the Paleogene, have never been recovered from Late Cretaceous to Oligocene strata across Gondwana. Instead, these regions were inhabited by temperate rainforests dominated by podocarps, Araucariaceae, Nothofagaceae, Proteaceae, and tree ferns (6, 7), which occupied niches potentially suitable for Fagaceae since the Late Cretaceous. Hence, the southern route hypothesis would require that generations of palynologists had overlooked the characteristic pollen of Castaneoideae in Gondwanan records. Third, Wilf et al. argue that the Patagonian fossil plant assemblages are similar to modern assemblages with “substantial Gondwanic history.” We note that a large part of modern Castanopsis distribution occurs outside living plant communities with substantial Gondwanan history. Modern plant communities with Castanopsis differ considerably in New Guinea, the Himalayas, and Japan (8).

Regarding (iii), molecular data reject the notion that “North American and European fossils assigned to Castanopsis [are] more distant relatives of the extant genus than are the new Argentine fossils” and are at odds with the southern route hypothesis. Nuclear data (9) show a sister relationship of Castanopsis with Castanea. Castanea-Castanopsis are close relatives of oaks, genus Quercus. Quercus was evolved and started to radiate by the early Eocene (10). Castanea-Castanopsis were already diverged when the North and South American Castanopsis-like fossils were deposited. Isolated biogeographic history inevitably would have left imprints in plastome signatures of Castanopsis; for instance, South American Nothofagaceae (subgenus Nothofagus) have different plastid signatures than their New Guinean–New Caledonian sister (subgenus Brassospora) despite potential long-distance dispersal (11). Within Nothofagus, three divergent, old plastid lineages indicate chloroplast capture and a larger distribution area in the past (12). Nuclear-plastid incongruence and strong geographic signal in the plastids is also found in core Fagaceae, which include all Castaneoideae and Quercus. If the Patagonian fossil represented the already diverged genus Castanopsis and if it were a precursor of modern-day Asian Castanopsis, one should find a divergent and genus-diagnostic plastid signature in at least some Castanopsis, with closer affinity to New World than to Old World Fagaceae. Shared plastid histories and near-identical plastid plant barcodes (matK, rbcL, atpB-rbcL, trnH-psbA) reflect shared biogeographic histories in the eastern hemispheric Quercus subgenus Cerris, Lithocarpus, Castanea, and Castanopsis (1315) and set them apart from the western hemispheric Quercus subgenus Quercus, Notholithocarpus, and Chrysolepis. On the basis of all available genetic data, modern Castanopsis evolved near Castanea, Lithocarpus, and the mainly subtropical Eurasian oaks (subgenus Cerris), all of which lack a fossil record outside Eurasia but existed during the Paleogene of Eurasia. Therefore, the range expansion into South America was a dead end in the biogeographic history of the Fagaceae.

In sum, we are excited by the finding of Wilf and colleagues, but without fossil (Antarctic Castaneoideae) and molecular (distinct plastid signature of Castanopsis) evidence, we do not see any evidence for the southern route as proposed by them.

*Difficult to assess on the basis of limited material.

References

Acknowledgments: Funding: Supported by a grant of the Swedish Research Council (VR; project no. 2015-03986) to T.D. Author contributions: T.D. wrote the first draft. All authors discussed and wrote the final manuscript. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper. For further information regarding the fossil record of Castaneoideae, please contact the corresponding author.
View Abstract

Stay Connected to Science

Navigate This Article