Occurrence of preanthesis cleistogamy in Richardia brasiliensis suggests it may be more common in Rubiaceae than realised

ABSTRACT

Background

Most Rubiaceae species are hermaphroditic with outcrossing often promoted by distyly and self-incompatibility, as well as by andromonoecy and dioecy in some species. However, cleistogamy has been reported in three genera, including Richardia.

Aims

To characterise the reproductive biology of Richardia brasiliensis (Rubiaceae), a ruderal herb native to southern USA, Central America and large parts of South America, and determine if it exhibits preanthesis cleistogamy.

Methods

In three natural populations of R. brasiliensis in Argentina, floral processes were monitored in the flower-bud stage and during anthesis until senescence. In addition, stigma receptivity and pollen viability were determined at these different floral stages in two populations, and experimental controlled pollinations were performed to check for spontaneous self-pollination, natural pollination and apomixis. Floral visiting insects were observed and collected for identification.

Results

Autonomous self-pollination occurred when flowers were in bud and stigmas were receptive, with ovules fertilised prior to anthesis. These flowers were protogynous and functionally cleistogamous, though later passed through a chasmogamous phase when flowers opened and were visited by diverse insects.

Conclusions

Our study demonstrates that preanthesis cleistogamy occurs in flower-buds of R. brasiliensis, indicating that this mode of self-pollination and self-fertilisation may be more common in Rubiaceae than previously realised.

KEY HIGHLIGHTS

1. Flowers of Richardia brasiliensis exhibit two striking features in the flower-bud stage: stigmatic receptivity and self-pollination causing preanthesis cleistogamy to occur.

2. Pollen grain germination on distinctly enlarged papillae cells and pollen tube growth in the style to the ovary were observed in 85% of fixed flower-buds.

3. During anthesis, flowers are visited by a diversity of insects collecting pollen and nectar (Orders Hymenoptera (Apidae, Formicidae and Trigonalidae) and Diptera (Syrphidae)), but due to preanthesis cleistogamy most reproduction occurs via self-fertilisation.

KEYWORDS:

• Plant breeding systems
• preanthesis cleistogamy
• prior self-fertilisation
• reproductive biology
• Richardia brasiliensis
• Rubiaceae

Acknowledgments

We thank Bernardo Holman; and Martin Sanchez and Conrado Holzer Park rangers of Parque Provincial San Cayetano for fieldwork support. Irene Caponio, Fac. de Agronomía (UNNE) helped us in the interpretation of phenology and development of the species. Our gratitude to the members of Laboratorio de Artrópodos (UNNE) for guiding us with species determination, to Dr. Adán Ávalos for floral visitors’ behaviour suggestions, and Guillermo Montero (FCA-UNR) for valuable bibliography. The authors thank three reviewers and the Subject Editor, our special gratitude to the late Dr. Richard Abbott, whose comments have greatly improved the manuscript. We are also very grateful to the late Dr. Peter E. Gibbs for his interest, encouragement and valuable advice at various stages of our work.

Disclosure statement

The authors confirm that there are no relevant financial or non-financial competing interests to report.

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/17550874.2024.2329741

Additional information

Funding

This work was supported by Consejo Interuniversitario Nacional (CIN) under Grant EVC-CIN Res. 361/16, “Biología reproductiva de Richardia brasiliensis (Spermacoceae, Rubiaceae) en poblaciones naturales del Parque Provincial San Cayetano, Corrientes”; and the Universidad Nacional del Nordeste under Grant PI 16P001: Filogenia, evolución, biogeografía, reproducción y taxonomía de Rubiáceas herbáceas de Sudamérica, con énfasis en especies arvenses.