1. Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, Colorado 80309, USA

Correspondence to: William E. Friedman¹ Correspondence and requests for materials should be addressed to W.E.F. (e-mail: Email: ned@colorado.edu).

The flowering plant family Hydatellaceae was recently discovered to be allied to the ancient angiosperm lineage Nymphaeales (water lilies)¹. Because of its critical phylogenetic position, members of the Hydatellaceae have the potential to provide insights into the origin and early diversification of angiosperms². Here I report that Hydatella expresses several rare embryological features that, in combination, are found only in members of the Nymphaeales. At maturity, the female gametophyte is four-celled, four-nucleate and will produce a diploid endosperm, as is characteristic of most early divergent angiosperm lineages^{3, 4}. As with all members of the Nymphaeales, endosperm in Hydatella is minimally developed and perisperm is the major embryo-nourishing tissue within the seed^{5, 6}. Remarkably, Hydatella exhibits a maternal seed-provisioning strategy that is unique among flowering plants, but common to all gymnosperms⁷: pre-fertilization allocation of nutrients to the embryo-nourishing tissue. This exceptional case of pre-fertilization maternal provisioning of a seed in Hydatella may well be an apomorphic feature of Hydatellaceae alone but, given the newly discovered phylogenetic position of this family, potentially represents a plesiomorphic and transitional condition associated with the origin of flowering plants from gymnospermous ancestors.

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