Utilizing Polyploidy for Developing Improved Nursery Crops: Restoring Fertility in Wide Hybrids, Limiting Fertility of Invasive Species, Embryo Culture of Triploids, Pest Resistance, and Inheritance of Ornamental Traits

Abstract

Multiple projects were conducted to investigate the potential for developing a breeding program utilizing species of Catalpa Scop. and Chilopsis D. Don. The efficacy of oryzalin was evaluated for inducing polyploidy and restoring fertility in the sterile, intergeneric hybrid xChitalpa tashkentensis Elias & Wisura [Catalpa bignonioides Walt. x Chilopsis linearis (Cav.) Sweet] 'Pink Dawn'. Submerging meristems in 150 μM oryzalin for up to 24 hours was effective at inducing tetraploids and cytochimeras. Pollen from the diploid cultivar was non-viable, but pollen from the polyploid stained and germinated as well as pollen from progenitor taxa. Polyploid xChitalpa were self-compatible yielding tetraploids when self pollinated and triploids when crossed with C. bignonioides, but reciprocal crosses with Chilopsis taxa failed. To increase recovery of triploids, we investigated germination of ovules and embryos at various harvest dates on Schenk and Hildebrandt (SH) basal salts supplemented with various medium components. Germination of triploid [(polyploid xChitalpa) x C. bignonioides] and tetraploid (selfed polyploid xChitalpa) embryos was greatest at 7 weeks after pollination on SH with 20 g•L⁺¹ sucrose and ≥ 1 μM gibberellic acid (GA₃). Triploids [Chilopsis linearis x (polyploid xChitalpa)] germinated < 5%. Additional studies were conducted to screen diverse taxa for resistance to powdery mildew (PM), Erysiphe elevata (Burr.) U. Braun & S. Takam, and catalpa sphinx larvae (CSL), Ceratomia catalpae (Boisduval). Twenty-four taxa from Catalpa (section Catalpa Paclt and Macrocatalpa Grisebach), Chilopsis, and xChitalpa were screened in 2004-05 for susceptibility to PM. Disease incidence and severity were recorded to calculate area under the disease progress curves (AUDPC) for each year. North American Catalpa spp. in sect. Catalpa, Chilopsis, and xChitalpa taxa were all moderate to highly susceptible to PM. Chinese Catalpa spp. in sect. Catalpa and West Indian sect. Macrocatalpa were resistant to PM. Hybrids among North American and Chinese Catalpa spp. in sect. Catalpa varied in susceptibility, indicating inheritance of partial resistance to PM. A no-choice feeding study conducted with CSL in 2005 found no differences in survival or growth of larvae reared on taxa from both sections of Catalpa, Chilopsis, and xChitalpa. Future breeding of xChitalpa can utilize the identified sources of resistance for PM; however, a source of resistance to CSL was not found. Triploids are generally infertile and may be deployed by breeders to limit invasive potential of introduced ornamentals. However, inheritance of ornamental traits can be complex at higher ploidy levels. Inheritance of two mutant foliage types, variegated and purple, was investigated for diploid, triploid and tetraploid tutsan (Hypericum androsaemum L.). Fertility of progeny was evaluated with pollen viability tests, percent fruit set, and germinative capacity of seed from specific crosses. Segregation ratios were determined for diploids in reciprocal di-hybrid F₁, F₂, BC[subscript 1P1], and BC[subscript 1P2] families and selfed F₂s with the parental phenotypes and triploid and tetraploid F₂s. Diploid di-hybrid crosses fit the expected 9:3:3:1 ratio for a single, simple recessive gene for both traits, with no evidence of linkage. A novel phenotype representing a combination of parental phenotypes was recovered. Data from backcrosses and selfing also supported the recessive model. Both traits behaved as expected at the triploid level; however, at the tetraploid level the number of variegated progeny increased, with segregation ratios between random chromosome and random chromatid assortment models. We propose the gene symbol var (variegated) and pl (purple leaf) for the variegated and purple alleles, respectively. Triploid pollen stained moderately well, but pollen germination was low. Triploid plants demonstrated extremely low male fertility and no measurable female fertility (no viable seed production). Research presented herein demonstrates the feasibility of manipulating ploidy levels for breeding desirable ornamental traits including non-invasiveness.