DIGITAL VISUALIZATION OF FOSSIL AND MODERN SEEDS PROVIDES INSIGHTS INTO PAST DIVERSITY AND EVOLUTION OF BANANAS AND GINGERS (ORDER ZINGIBERALES)

The Zingiberales (bananas, gingers, and relatives) are an economically and ecologically important group of tropical to subtropical monocot plants that today consist of eight families and ca. 2500 species. Zingiberales fossils extend from the late Cretaceous to the Pliocene of North and Central America, India, and Eurasia. As fossil pollen of Zingiberales does not preserve, and these are non-woody plants, a good understanding of leaf, fruit, and seed morphology is critical for recognizing the broad temporal and spatial patterns of evolution in this group. Zingiberales appear to have undergone an early, rapid radiation as documented by molecular and morphological data. The oldest zingiberalean is Spirematospermum, known from fruits and seeds. However, there is disagreement about the closest affinities of this genus: is it with the basal banana family (Musaceae) or the more derived ginger family, Zingiberaceae? Synchrotron-based X-ray tomographic microscopy (SRXTM) was used to examine Spirematospermum and over 80 extant species from all eight families in order to identify phylogenetically significant characters. Standard histological techniques are ineffective on zingiberalean seeds because of their hard seed coats. However, SRXTM allows non-destructive visualization of both gross morphology and detailed features of internal structure, providing much data within a single scan. SRXTM reveals that a chalazal chamber – present in Spirematospermum, thought to be diagnostic of Musaceae, and present but reduced in Costaceae – is also present in tribe Alpiniae (Alpinioideae; Zingiberaceae), and thus cannot be used as evidence for placing Spirematospermum in Musaceae as done in previous studies. SRXTM also provides important information on operculum morphology, embryo shape, and seed coat anatomy. Our results indicate that Spirematospermum shows a mosaic of musaceous and zingiberaceous features, suggesting that i) Musaceae and Zingiberaceae are more closely related than currently accepted phylogenetic trees indicate or ii) Spirematospermum represents an extinct family-level lineage within Zingiberales. These implications can be further tested with future studies on phylogenetic relationships of the group and the incorporation of seed anatomical data from other extinct genera.