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17 Pag., 1 Tabl., 7 Fig. The definitive version is available at: http://www.pnas.org/ ; Although the biological function of fruiting is the production and dissemination of seeds, humans have developed seedless fruits in a number of plant species to facilitate consumption. Here we describe a unique spontaneous seedless mutant (Thai seedless; Ts) of Annona squamosa (sugar apple), a member of the early-divergent magnoliid angiosperm clade. Ovules (seed precursors) of the mutant lack the outer of two normal integuments, a phenocopy of the inner no outer (ino) mutant of Arabidopsis thaliana. Cloning of the INO ortholog from A. squamosa confirmed conservation of the outer integument-specific expression pattern of this gene between the two species. All regions of the gene were detectable in wild-type A. squamosa and in other members of this genus. However, no region of the INO gene could be detected in Ts plants, indicating apparent deletion of the INO locus. These results provide a case of a candidate gene approach revealing the apparent molecular basis of a useful agronomic trait (seedless fruit) in a crop species, and indicate conservation of the role of a critical regulator of ovule development between eudicots and more ancient lineages of angiosperms. The outer integument is one synapomorphy of angiosperms separating them from other extant seed plants, and the results suggest that the evolution of this structure was contemporaneous with the derivation of INO from ancestral YABBY genes. Thus, a unique lateral structure appears to have coevolved with a novel gene family member essential for the structure's formation. ; This work was supported by grants from the Spanish Ministry of Science and Innovation (Project Grants AGL2007- 60130/AGR, AGL2009-12621, and AGL2010-2140), Instituto Nacional de Investigación Agraria y Alimentaria (RF2009-00010), GIE-Aragón 43, Junta de Andalucía (FEDER AGR2742), and the European Union under the INCODEV Program (Contract 015100) and from the US National Science Foundation (IOS-0920618). J.L. was supported by grants from Junta de Andalucía and the National Science Foundation Molecular and Organismic Research in Plant History (MORPH) Network. ; Peer reviewed

37 Pag., 1 Tabl., 6 Fig. The definitive version is available at: http://aob.oxfordjournals.org/ ; Background and Aims An intense pollen–pistil interaction precedes fertilization. This interaction is of particular relevance in agronomically important species where seeds or fruits are the edible part. Over time some agronomically species have been selected for the ability to produce fruit without seeds. While this phenomenon is critical for commercial production in some species, very little is known about the events behind the production of seedless fruit. In this work, the relationship between pollen–pistil interaction and the onset of fruiting was investigated in citrus mandarin. Methods Pistils were sequentially examined in hand-pollinated flowers paying attention to pollen-tube behaviour, and to cytochemical changes along the pollen-tube pathway. To evaluate which of these changes were induced by pollination/fertilization and which were developmentally regulated, pollinated and unpollinated pistils were compared. Also the onset of fruiting was timed and changes in the ovary examined. ; Key Results Conspicuous changes occurred in the pistil along the pollen-tube pathway, which took place in a basipetal way encompassing the timing of pollen-tube growth. However, these changes appear to be developmentally regulated as they happened in the same way and at the same time in unpollinated flowers. Moreover, the onset of fruiting occurred prior to fertilization and the very same changes could be observed in unpollinated flowers. Conclusions Pollen–pistil interaction in citrus showed similarities with unrelated species and families belonging to other taxa. The uncoupling of the reproductive and fruiting processes accounts for the parthenocarpic ability of unpollinated flowers to produce fruit in citrus. However, the maintenance of a functional reproductive process reflects the potential to produce seeded fruits, providing a basis for the understanding of the production of seeded or unseeded fruits and further understanding of the process of parthenocarpy in other species. ; This work was supported by: the Italian Ministry of the University – Project PRIN "The productive process in fruit tree species: molecular, physiological and agronomical aspects of floral incompatibility and strategies for its control"; the Minister of Science and Innovation – EU Feder [Project grant CICYT AGL-2006-13529-CO2-01/AGR and AGL-2009-12621-CO2-01/AGR]; the Aragon Government A-43 support to the group. ; Peer reviewed