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(American Journal of Botany. 2009;96:802-808.) doi: 10.3732/ajb.0800167 © 2009 Botanical Society of America, Inc.

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The coexistence of bicellular and tricellular pollen in Annona cherimola (Annonaceae): Implications for pollen evolution¹

² Estación Experimental "La Mayora", CSIC 29760 Algarrobo-Costa, Málaga, Spain ³ Dept. Pomology, Estación Experimental "Aula Dei", CSIC, Apdo. 202/ 50080 Zaragoza, Spain

ABSTRACT

Most angiosperms release bicellular pollen. However, in about one-third of extant angiosperms, the second pollen mitosis occurs before anthesis such that pollen is tricellular upon release. The shift from bicellular to tricellular development has occurred several times independently, but its causes are largely unknown. In this work, we observed the coexistence of both kinds of pollen at anther dehiscence in Annona cherimola, a species that belongs to the basal angiosperm family Annonaceae. Examination of pollen cell number during anther development showed that this coexistence was due to a late mitosis starting shortly before pollen shedding. Both types of pollen germinated equally well over the course of development. Because variable proportions of bicellular and tricellular pollen were observed at different sampling times, we tested the role of temperature by performing field and growth chamber experiments, which showed that higher temperatures near anthesis advanced the time of pollen mitosis II. The results show that selection could favor the production of tricellular pollen under certain environmental circumstances that prime rapid pollen germination and provide evidence of a system in which developmental variation persists, but that can be modified by external factors such as temperature.

Key Words: Annona cherimola • Annonaceae • bicellular pollen • cherimoya • pollen evolution • tricellular pollen

Received for publication 12 May 2008. Accepted for publication 13 January 2009.

FOOTNOTES

¹ Financial support for this work was provided by the Spanish Ministry of Education (Project grants AGL2004-02290/AGR, AGL2006-13529 and AGL2007-60130/AGR), GIC-Aragón 43, Junta de Andalucía (AGR2742), and the European Union under the INCO-DEV program (Contract 015100). J.L. was supported by a grant from Junta de Andalucía. The authors thank P. Rudall and an anonymous reviewer for helpful comments on the manuscript, and M. C. Risueño and P. S. Testillano for help with analyses involving the confocal microscope.

⁴ Author for correspondence (e-mail: ihormaza{at}eelm.csic.es)

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