Refined interpretation of the pistillate flower in Ceratophyllum sheds fresh light on gynoecium evolution in angiospermsстатья
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Аннотация:Molecular phylogenetic analyses have revealed a superclade of mesangiosperms with fiveextant lineages: monocots, eudicots, magnoliids, Ceratophyllum and Chloranthaceae. BothCeratophyllum and Chloranthaceae are ancient lineages with a long fossil record; their preciseplacement within mesangiosperms is uncertain. Morphological studies have suggested thatthey form a clade together with some Cretaceous fossils, including Canrightia, Montsechia andPseudoasterophyllites. Apart from Canrightia, members of this clade share unilocular gynoeciacommonly interpreted as monomerous with ascidiate carpels. Alternatively, the gynoecium ofCeratophyllum has also been interpreted as syncarpous with a single fertile carpel(pseudomonomerous). We investigate patterns of morphological, anatomical anddevelopmental variation in gynoecia of three Ceratophyllum species to explore thecontroversial interpretation of its gynoecium as either monomerous orpseudomonomerous. We use an angiosperm-wide morphological data set andcontrasting tree topologies to estimate the ancestral gynoecium type in bothCeratophyllum and mesangiosperms. Gynoecia of all three Ceratophyllum speciespossess a small (sometimes vestigial) glandular appendage on the abaxial side and anoccasionally bifurcating apex. The ovary is usually unilocular with two procambium strands, butsometimes bilocular and/or with three strands in C. demersum. None of the possiblephylogenetic placements strongly suggest apocarpy in the stem lineage of Ceratophyllum.Rescoring Ceratophyllum as having two united carpels affects broader-scale reconstructionsof the ancestral gynoecium in mesangiosperms. Our interpretation of the glandular appendageas a tepal or staminode homologue makes the Ceratophyllum ovary inferior, thus resembling(semi)inferior ovaries of most Chloranthaceae and potentially related fossils Canrightia andZlatkocarpus. The entire structure of the flower of Ceratophyllum suggests strong reductionfollowing a long and complex evolutionary history. The widely accepted notion that apocarpy isancestral in mesangiosperms (and angiosperms) lacks robust support, regardless of whichmodes of carpel fusion are considered. Our study highlights the crucial importance ofincorporating fossils into large-scale analyses to understand character evolution.