Intersectional Hybridization among Trichostema Species (2024)

Keywords: ornamental plant breeding; blue curls; interspecific hybrid; intersectional hybrid; hybrid sterility

Materials and Methods

Results and Discussion

Five viable seedlings were obtained from the T. arizonicum (female) × T. lanatum (male) cross, of which two later died. One viable seedling was obtained from the T. purpusii (female) × T. lanatum (male) cross. For both crosses, no seeds resulted when T. lanatum was used as the female. This could be from the lack of pollen germination or insufficient pollen tube growth through the highly exserted style found on T. lanatum compared with the relatively shorter styles on T. arizonicum and T. purpusii.

Keeley and Fotheringham (1998) reported that T. lanatum seed would not germinate without coupling a year-long soil contact and smoke treatment, resulting in ≈30% germination. The smoke, having the active compound butenolide 3-methyl-2H-furo[2,3-c]pyran-2-one, is thought to allow a germination block to be overcome (Flematti et al., 2004). No smoke or year-long soil contact treatment was administered to either group of hybrid seed (≈15 seeds total), and six seedlings resulted, indicating that extended soil and smoke treatments may be species specific. A smoke treatment may have increased the germination percentage. After initial seedlings had developed, a smoke treatment was applied to ungerminated seeds, with no success.

The T. purpusii × T. lanatum hybrid, which flowered for the first time on 14 May 2007, unfortunately did not retain the unique pink corolla lobe color of T. purpusii. Corolla lobes were actually violet, which is intermediate in color between the two parents. This plant did show hybrid vigor in the anterior corolla lobe, as its size was greater than either parent (Fig. 1). Leaf, stamen, and pistil size were intermediate between the parents (Fig. 1; Table 1). Leaf and pistil morphology were also intermediate for the T. arizonicum × T. lanatum hybrids, which flowered for the first time 15 May 2005; however, stamen length was shorter than either parent (Fig. 2; Table 1). Corolla lobe color for T. arizonicum × T. lanatum hybrids were purple, and thus resemble the purple-blue flowers of T. lanatum.

The T. purpusii × T. lanatum hybrid did set limited seed, whereas no seed has been produced by T. arizonicum × T. lanatum hybrids for two consecutive years despite attempts to self and sib-cross the hybrids. The T. arizonicum × T. lanatum hybrids were observed to have limited pollen production. Anthers were nondehiscent and uncharacteristically white, turning to brown upon maturity. Similar intermediate male sterile flowers, having white indehiscent anthers that turn brown, unlike fertile purple anthers, were noted in Rosmarinus officinalis L. (Ubera-Jiménez and Hidalgo-Fernández, 1992).

Microscopic examination revealed that pollen from T. arizonicum × T. lanatum hybrids were roughly half the size of pollen from either parent. Additionally, the pollen of T. arizonicum × T. lanatum hybrids were flaccid, ranging from elliptical to crescent-shaped, indicating lack of maturity (Fig. 3, Table 2). Sarkissian and Harder (2001) indicated that amount and quality of pollen, including size and morphology, are often important components of fitness and relate to mating success. Attempts to germinate pollen in vitro on Brewbaker and Kwack (1963) media were not successful.

The lack of fertility in the F1 plants from T. arizonicum × T. lanatum crosses was not expected, as both plants are reported to have the same chromosome number (Lewis, 1960). It is known that hybrids resulting from taxonomically unrelated parents (such as those from different sections) tend to be sterile or of low fertility (Qureshi et al., 2002). Sterile hybrids were also seen in the genus Calylophus Spach., where the high degree of sterility in certain intrasectional crosses was found to be correlated with cytological differences between the parental plants (Towner, 1977).

In an effort to have fertile plants for future breeding, a solution of 7 μm oryzalin was applied as a mist using a spray bottle on a few emerging seedlings, which were derived from repeating earlier crossing experiments between T. arizonicum and T. lanatum. Inducing polyploidy is a viable method for improving meiotic pairing and restoring fertility in wide crosses (Sanford, 1983). A single plant (likely mixoploid) resulted from this study. The main stem on this plant branched, so that half of the plant produced branches with flowers that had enhanced flower size, corolla lobe color, stamen length, pollen production, and fertility (Fig. 4). Crosses between this oryzalin-induced hybrid and T. lanatum produced nutlets, indicating that the sterility seen in the original T. arizonicum × T. lanatum crosses was likely from a meiotic imbalance.

Propagation of the hybrids was not difficult, as softwood cuttings rooted at high percentages within 2 to 3 weeks (data not shown). We were also successful in propagating both hybrids by plant tissue culture. By establishing plants in tissue culture, large numbers of plants can be generated quickly without having to maintain a large number of stock plants.

The taxonomic significance of the intersectional hybrids T. arizonicum × T. lanatum and T. purpusii × T. lanatum provides genetic evidence that the genomes are sufficiently hom*ologous to permit the formation of hybrids. In the case of T. purpusii × T. lanatum, these hybrids are fertile. The sterility and later restored fertility seen in the T. arizonicum × T. lanatum hybrids indicate that these two taxa are less related. We can also conclude that although T. arizonicum and T. purpusii are found in different sections than T. lanatum, these species may be more closely related than is apparent from their present classification in distinct sections. This goes against Huang (2002), who found that sect. Chromocephalum is a sister group to the remainder of the genus after she combined morphological and molecular data. These intersectional hybrids may be viewed as evidence for the feasibility of crossing other Trichostema species, having similar chromosome numbers, within and among sections. The fact that oryzalin was successful in restoring fertility further expands the breeding potential among wide crosses within Trichostema. An important result of continued hybridization studies may well be a greatly improved understanding of the taxonomy of this genus. The data we collected agrees with Spira (1978) who reported that flower size and reproductive parts vary greatly within Trichostema species. The T. purpusii × T. lanatum hybrid has not been selected for release as of yet; however, one T. arizonicum × T. lanatum hybrid was selected and released as ‘Blue Myth’ (Dunn and Lindstrom, 2007).