Occurrence of Unreduced Female Gametes Leads to Sexual Polyploidization in Lantana

Lantana camara, a member of the verbena family, is a popular ornamental yet highly invasive plant. It can escape from cultivation through seed dispersal and can contaminate native lantana species (Lantana depressa) through cross-pollination. Ploidy manipulation is being used as a genetic approach to produce sterile, noninvasive lantana cultivars. Polyploids have been observed in lantana (Lantana), but little information is available about the mechanisms for lantana polyploidization and the possible effects of natural polyploidization on the sterility (or fertility) of lantana triploids. In this study, we analyzed the ploidy level of more than 1500 lantana progeny from self, open, and/or controlled pollinations of 10 commercial cultivars and seven breeding lines. Our results confirmed the occurrence of unreduced gametes, specifically, unreduced female gametes (UFGs), in lantana. The frequency of UFG formation varied among commercial cultivars, and cultivars/breeding lines could be categorized into two groups: UFG producers and nonproducers. Tetraploid cultivars Gold, Pink Caprice, and Radiation fall into the UFG-producing group, while diploid cultivars Cream, Denholm White and Lola and tetraploid cultivars Carlos, Dallas Red and Irene belong to the nonproducer group. The frequency of UFG formation observed in nine UFG producers was 5.5% to 100%, varying with cultivar, growing condition, and/or pollination scheme. Progeny of the cross between ‘Carlos’ (seed parent) and ‘Gold’ (pollen parent) also showed the ability to produce UFGs, indicating that the trait (UFG formation) could be transmitted from ‘Gold’ to its progeny and is likely to be controlled by nuclear gene(s). Lantana triploids with or without the UFG-forming ability in its genetic background showed a significant difference in seed set: the former produced abundant seed when pollinated, while the latter produced little or no seed. The results stress the need to avoid using lantana with UFG-forming ability as parents in crosses designed to produce sterile triploids for invasiveness control. Additionally, the results from this study suggest multiple pathways for emergence and evolution of polyploids in cultivated and naturalized lantana populations.