Cryptic self-incompatibility in tristylous Decodon verticillatus (Lythraceae)

Area of Science:

• Plant reproductive biology
• Evolutionary botany
• Population genetics

Background:

• Many self-compatible plants regulate self-fertilization using postpollination mechanisms.
• Cryptic self-incompatibility (CSI) confers a siring advantage to cross pollen over self pollen.
• Previous studies in Decodon verticillatus showed a cross-pollen advantage, but the role of prezygotic vs. postzygotic factors was unclear.

Purpose of the Study:

• To investigate the role of prezygotic mechanisms in the siring advantage of cross pollen in the self-compatible, tristylous plant Decodon verticillatus.
• To differentiate between prezygotic discrimination and early inbreeding depression as causes for the observed siring advantage.

Main Methods:

• Pollen tube numbers were quantified at different time points (24, 36, and 48 hours) after self- and cross-pollination in a natural population.
• Comparisons were made between legitimate and illegitimate cross-pollinations.

Main Results:

• Cross pollen produced nearly twice as many pollen tubes as self pollen within 24 hours post-pollination.
• The difference in pollen tube numbers diminished by 36 and 48 hours, indicating differential germination or growth rates, not pollen tube attrition.
• No difference in pollen tube numbers was observed between legitimate and illegitimate cross-pollinations.

Conclusions:

• Prezygotic mechanisms, specifically differential pollen tube growth, contribute to the siring advantage of cross pollen in Decodon verticillatus.
• The observed CSI in D. verticillatus is independent of heteromorphic self- and intramorph-incompatibility systems found in other Lythraceae.
• This CSI mechanism likely balances the minimization of selfing when compatible pollen is abundant with the maximization of fecundity during clonal spread when compatible pollen is scarce.