Morphology of Hydatellaceae, an anomalous aquatic family recently recognized as an early‐divergent angiosperm lineage

Abstract

The family Hydatellaceae was recently reassigned to the early-divergent angiosperm order Nymphaeales rather than the monocot order Poales. This dramatic taxonomic adjustment allows comparison with other early-divergent angiosperms, both extant and extinct. Hydatellaceae possess some monocot-like features that could represent adaptations to an aquatic habit. Ecophysiological parallels can also be drawn from fossil taxa that are known from small achene-like diaspores, as in Hydatellaceae. Reproductive units of Hydatellaceae consist of perianthlike bracts enclosing several pistils and/or stamens. In species with bisexual reproductive units, a single unit resembles an "inside-out" flower, in which stamens are surrounded by carpels that are initiated centrifugally. Furthermore, involucre development in Trithuria submersa, with delayed growth of second whorl bracts, resembles similar delayed development of the second perianth whorl in Cabomba. Several hypotheses on the homologies of reproductive units in Hydatellaceae are explored. Currently, the most plausible interpretation is that each reproductive unit represents an aggregation of reduced unisexual apetalous flowers, which are thus very different from flowers of Nymphaeales. Each pistil in Hydatellaceae is morphologically and developmentally consistent with a solitary ascidiate carpel. However, ascidiate carpel development, consistent with placement in Nymphaeales, is closely similar to pseudomonomerous pistil development as in Poaes.

Tracing the early evolutionary diversification of the angiosperm flower

“…Th is state persists to the base of the mesangiosperms ; because the perianth was whorled and trimerous by this point, this implies that the common ancestor of mesangiosperms had three or more whorls of three tepals. Th e inference that the perianth was lost in Hydatellaceae assumes that the superfi cially fl ower-like reproductive units of this group are infl orescence s with basal bract s and unisexual fl owers consisting of one stamen or one carpel (Endress and Doyle, 2009 ), rather than incompletely organized 'pre-fl owers' (one possibility considered by Rudall et al, 2007 ) or 'non-fl owers' . But even if the infl orescence interpretation is incorrect, it would be most parsimonious to assume that the lack of typical fl oral organization in Hydatellaceae was derived rather than primitive.…”

Integrating Early Cretaceous Fossils into the Phylogeny of Living Angiosperms: ANITA Lines and Relatives of Chloranthaceae

“…Considering unequivocal synapomorphies that support these results (derived states unambiguously placed on the branch in-563 dicated), Monetianthus is linked with Cabombaceae plus Nymphaeaceae by more than two ovules per carpel (112), with Nymphaeaceae by a ring of more than five carpels (96) and eusyncarpy (106), and with Barclaya and Nymphaeoideae by inferior ovary (48) and globose pollen (83) versus superior ovary and boat-shaped pollen in Nuphar (for sources of data on Recent taxa, see Material and Methods). Another synapomorphy of Monetianthus and Nymphaeales is laminar placentation (113), in which we include both the typical laminar placentation of Nymphaeaceae and related conditions (such as "dorsal") in Cabombaceae, but where this feature arose is equivocal, because the position of the single apical ovule in Trithuria (pHydatellaceae) is uncertain (Rudall et al 2007) and was therefore scored as unknown. The positions with Barclaya and Nymphaeoideae alone conflict with the monosulcate pollen of Monetianthus, since the two living groups are united by zonasulculate pollen (84), but the extra step in this character is balanced by the fact that Monetianthus shares medium-sized pollen (82) with Barclaya and a protruding floral apex (52) with Nymphaeoideae.…”

Ancestral traits and specializations in the flowers of the basal grade of living angiosperms

“…(2007) Fig. 7B; Hamann, 1975Hamann, , 1976Rudall & al., 2007). It has been suggested that these structures may represent a primitive, pre floral or "nonfloral" state (Rudall & al., 2009), but parsimony optimization implies instead that they are derived.…”