Lantana is a member of the Verbenaceae and has been widely grown as a container plant, hanging basket plant, groundcover, hedge, or accent plant (Howard, 1969). It is popular in the nursery industry because of easy propagation and short production cycles, and is popular among gardeners because it attracts butterflies and is tolerant to droughts and poor soil conditions. Many nursery growers produce lantana, especially in the southern United States. For example, a survey of the Florida nursery industry, which consists of more than 5000 nurseries, indicates that 19% of the responding nurseries grew lantana and that the annual sales value in Florida alone was over $40 million (Wirth et al., 2004). The majority of the commercial lantana cultivars belong to Lantana camara. This species can escape from cultivation through seed dispersal and can invade agricultural and natural lands and hybridize with native lantana species (Lantana depressa). Because of these behaviors, L. camara has been listed as an invasive species in southern and central Florida (Florida Exotic Pest Plant Council, 2007). In several other countries including Australia, India, and South Africa, lantana is considered as a noxious weed or an invasive species (Sharma et al., 2005). Polyploid manipulation, particularly triploid production, has been proposed as a genetic approach to develop sterile, noninvasive lantana cultivars (Czarnecki et al., 2008). Similar genetic approaches (polyploid production and selection) are being used to sterilize other ornamental plants for invasiveness control (Ranney, 2004).
Polyploids are common in L. camara. Triploids, tetraploids, pentaploids, and hexaploids have been reported in cultivated and naturalized L. camara (Czarnecki et al., 2008; Natarajan and Ahuja, 1957; Spies and Stirton, 1982). Several lines of evidence suggest that polyploidization may be associated with the species’ invasive behavior (Sanders, 2001): Tetraploids are rare in the native populations of this species in tropical America, but are very common in the naturalized populations in Australia, India, and South Africa. Overall, tetraploids have courser leaves, grow more vigorously, and set more seeds (Sanders, 2001), whereas diploids tend to be dwarf and stunted. Tetraploids have a much wider range of distribution than diploids, and pentaploids are frequently found at high altitudes (Sinha and Sharma, 1984).
In plants, natural polyploidization can occur through somatic chromosome doubling or gametic nonreduction (Bretagnolle and Thompson, 1995). The former results from mitotic abnormalities in somatic (zygotic and meristematic) cells, while the latter results from meiotic abnormalities during gamete or gametophyte genesis and the formation of unreduced gametes (pollen and/or eggs). When an unreduced gamete unites with another unreduced gamete (bilateral) or with a normal haploid gamete (unilateral), the union leads to sexual polyploidization. To a certain extent, polyploidization via somatic chromosome doubling bears similarity to inbreeding, while polyploidization via unreduced gametes can retain heterozygosity. Thus, the two polyploidization processes can have significant differences in terms of genetic and evolutionary consequences to polyploidized species (Bretagnolle and Thompson, 1995; Hermsen, 1984).
Little information is available regarding the origin of polyploids in L. camara, except for a report by Khoshoo and Mahal (1967). The authors observed several tetraploids and a pentaploid in the open-pollinated (OP) progeny of a triploid and two hexaploids in the OP progeny of a pentaploid. They inferred that these tetraploid and hexaploid progeny with chromosome numbers higher than their parents must have come from union of unreduced female gametes with normally reduced pollen. The occurrence of UFGs has been reported in several plant species (Ramsey and Schemske, 1998; Stelly and Peloquin, 1986). Unreduced pollen has been reported in many plants and seems to be a more common mode of sexual polyploidization in plants (Bretagnolle and Thompson, 1995). Several studies have examined the pollen size and morphology of Lantana, but none of them reported the occurrence of unreduced pollen in L. camara (Raghavan and Arora, 1960; Sanders, 1987). In a recent pollen viability study, we examined tens of thousands of pollen grains and did not notice highly variable viable pollen grains within cultivars (D. Czarnecki and Z. Deng, unpublished).
During the course of interploid pollination and triploid generation, we observed pentaploid progeny from a tetraploid by diploid cross, which seems to indicate the occurrence of UFGs in the lantana cultivars used. Therefore, a study was undertaken to confirm the occurrence of UFGs in lantana, to determine its frequency and distribution in major commercial lantana cultivars, to test its transmissibility from generation to generation, and to determine if this trait would affect seed set on lantana triploids. Toward these objectives, progeny of commercial cultivars from self-pollination (SP) and OP were first analyzed for ploidy levels, followed by controlled pollinations among cultivars and breeding lines and ploidy analysis of their progeny. This report presents the results from these pollinations and ploidy analyses. Based on the results, we further propose a model for the origin of the multiple levels of polyploids in cultivated (and naturalized) L. camara and discuss the possible mechanisms for the formation of unreduced gametes in lantana and potential implications of this trait for lantana ploidy manipulation, particularly triploid generation and selection for sterile lantana development.
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