Ploidy level influences pollen tube growth and seed viability in interploidy crosses of Hydrangea macrophylla

Authors: Alexander, Lisa

Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/22/2020
Publication Date: 2/19/2020
Citation: Alexander, L.W. 2020. Ploidy level influences pollen tube growth and seed viability in interploidy crosses of Hydrangea macrophylla. Frontiers in Plant Science. https://doi.org/10.3389/fpls.2020.00100.
DOI: https://doi.org/10.3389/fpls.2020.00100

Interpretive Summary: Hydrangea macrophylla is one of the most important floral and nursery crops worldwide. Triploid H. macrophylla have desirable traits such as thicker stems, larger flowers, and more compact size compared to related diploids. However, it is unknown whether controlled crosses between diploid and triploid hydrangeas can be used to develop triploid varieties. In order to elucidate breeding mechanisms in hydrangea and produce hydrangeas with novel traits, an ARS scientist in McMinnville, TN crossed diploid and triploid H. macrophylla cultivars and observed pollen tube growth, seed set, seed germination, seedling growth, flowering, and ploidy level of progeny. Progeny of interploid crosses were diploid and aneuploid; no triploid progeny were recovered from crosses using triploid parents. Seedling germination and progeny growth rates were higher when the triploid was used as the seed rather than the pollen parent. Fertility of progeny was not reduced compared to parent cultivars, indicating progeny can be used to continue breeding. The progeny from interploid hybridizations have broad potential in breeding programs to provide genetic and phenotypic variability for the production of novel varieties.

Technical Abstract: All H. macrophylla cultivars tested to date are diploid (2n=2x=36) or triploid (2n=3x=54) and triploid H. macrophylla have thicker stems, larger flowers, and larger stoma compared to related diploids. It is unknown whether interploidy crosses between diploid and triploid hydrangeas can be used to develop triploid varieties. The objective of this study was to compare pollen tube development, fruit formation, and seed viability among intra- and interploidy pollinations of Hydrangea macrophylla and evaluate the genome size, pollen and pollen viability of resultant progeny. By 24 h post-pollination, pollen tubes had reached the ovaries of diploid and triploid flowers in 48.7% and 8.7% of samples, respectively ('2 = 30.6, p < 0.001). By 48 h post-pollination pollen tubes reached the ovaries of diploid and triploid flowers in 72.5% and 53.8% of samples, respectively ('2 = 26.5, p = 0.001), with no difference by 72 h post-pollination ('2 = 7.5, p = 0.60). Analysis of covariance showed that pollen tube length at 24 and 48 h post-pollination was significantly influenced by ploidy and flower length of the female parent. Progeny of interploidy crosses were diploid and aneuploid; no triploid progeny were recovered from crosses using triploid parents. Mean genome sizes of offspring from each cross type ranged from 4.4 pg for 2n x 2n progeny to 5.2 pg for 3n x 3n progeny. Estimated ploidy of progeny ranged from 2n for 2n x 2n crosses to 2n + 5 for 3n x 3n crosses. Pollen stainability rates of flowering progeny ranged from 69.6% to 76.4%. The progeny from interploidy hybridizations have broad potential in breeding programs to provide genetic and phenotypic variability for the production of novel varieties.