ASHS 2025 Conference

The Vitis genus consists of two subgenera, Vitis and Muscadinia, which are both cultivated for wine, juice and fresh consumption. Fresh market breeding programs for grapes (V. vinifera) and muscadines (M. rotundifolia) aim to develop stenospermocarpic seedless and perfect-flowered cultivars. Despite chromosome differences and genetic divergence between the Vitis (2n=38) and Muscadinia (2=40) subgenera, stenospermocarpic seedlessness from V. vinifera was successfully introgressed into M. rotundifolia through conventional breeding. Currently, molecular markers for stenospermocarpy have not been developed or validated in Muscadinia and diverse Vitis accessions. Before the divergence of the Vitis and Muscadinia subgenera, two mutations occurred in perfect flowered ancestors, leading to male and female sterility and resulting dioecy in wild plants from both subgenera. Kompetitive Allele Specific PCR (KASP) markers have been recently developed targeting the known causal mutations resulting in male sterility (VviINP1) and stenospermocarpy (VviAGL11) in Vitis. The markers named, seedless_Arg197Leu_site56.fas and female_INP_indel_site56.fas, were evaluated for their diagnostic potential across Vitis and Muscadinia species through sequence alignments with published genomes. The predictive ability of these markers was assessed using a validation panel from the University of Arkansas Division of Agriculture Fruit Breeding program, which included 918 seedlings from Vitis x Muscadinia hybrid breeding populations. Additionally, a range of diverse Vitis and Muscadinia accessions with known phenotypes were included to enhance understanding of marker effectiveness, including cultivars, selections, and wild material from USDA germplasm repositories and public and private breeding programs. In total, 209 diverse accessions were assessed with the seedless marker, and 320 accessions were evaluated with the flower sex marker. The evaluation for seedlessness and flower sex took place over 2023 and 2024. Excluding incomplete phenotype and genotype data, the stenospermocarpic marker (seedless_Arg197Leu_site56.fas) accurately predicted seedlessness in 921 of 924 entries. Additionally, 148 out of 203 seedlings that did not produce fruit in both growing seasons were predicted to be stenospermocarpic. A t-test comparing vine caliper between seeded and seedless material (P =0.178) showed no difference in vigor between the seeded and seedless vine and suggested that the high number of seedless vines with no fruit may be caused by sterility factors linked to the introgressed Vitis stenospermocarpy locus. The flower sex marker (female_INP_indel_site56.fas) correctly predicted flower sex in 1,137 of 1,138 entries. Overall, the KASP markers showed outstanding predictive performance, achieving accuracy rates of 99.9% for flower sex and 99.7% for seedlessness across Vitis, Muscadinia, and hybrid germplasm.