Origin of the outer integument and its implication for the angiosperm evolution

Abstract

Summery: The origin of the outer integument, which is one of the prominent synapomorphies of angiosperms, was inferred to explore the phylogenetic origin of the angiosperms. Studies based on the ovules of“ANITA”, a group of early-diverged families, revealed that the ovules with the cup-shaped outer integument are derived from the ovules with hood-shaped outer integument. The morphological marker in the seed was sought for from which the outer integument morphology could be inferred because the seeds with more fossilized potential than flowers would be helpful in deducing the primitive morphology of the outer integument based on the fossil records. As a result, I found that the relative position of the micropyle to the hilum could be used as the marker for inferring the outer integument morphology(i. e., hood-shaped or cup-shaped). Based on this marker, it is inferred that the oldest seed fossils of angiosperms are derived from the ovules with the hood-shaped outer integument. Neo- and Paleo- botanical data suggests that the hood-shaped outer integument with bilateral symmetry is primitive. The bilaterally symmetrical nature is one of the characteristics of the leaf, thus the outer integument could be homologous to the leaf. Further to examine this hypothesis I analyzed the expression pattern of INO homologue of Nymphaea alba as molecular marker for the dorsiventrality. INO homologue was limitedly expressed in the outer epidermis of the outer integument, suggesting that the outer integument has the dorsiventrality. These results imply that the outer integument is derived from the leaf.In addition to this, the carpel is suggested to be homologous to sporophyll, thus the gynoecium of angiosperms is compared to the unitegmic orthotropous ovule enclosed in two leaf-like organs, which is known in some groups of the seed ferns.