Proliferating seed cones in Metasequoia glyptostroboidesHu & Cheng (Cupressaceae s.l., Coniferales) elucidate the evolution of seed cones and ovules in Cupressaceae s.l.

Abstract

Proliferating seed cones are frequent in extant conifers. However, most proliferations are not useful for elucidating evolutionary scenarios, representing only chaotic mutations or a proliferation of the cone axis. In contrast the proliferating cones in Metasequoia studied here do not show a simple modification of a well understood pattern but show a known pattern with features that do not normally appear. A correct interpretation might allow a new hypothesis about the evolution of seed cones and ovules in conifers. In these proliferated Metasequoia seed cones several cone scales carried axillary, leafy short shoots instead of ovules. In most Cupressaceae, the shoot subtending the ovule – the so called seed scale – can no longer be detected. In proliferated seed cones a leafy short shoot replaces the ovules. This kind of proliferation can be regarded as an atavism, assuming that the sterile leaves on this short shoot represent transformed sterile ovules. This hypothesis is supported by the fact that the ovules in the axil of cone scales and the leaves formed on short shoots show a similarly centripetal or acropetal developmental pattern.