Distant hybridization, the crossing of different species or genera, holds immense potential for enriching genetic diversity and introducing desirable traits into cultivated crops. This comparative study delves into the challenges and successes of distant hybridization in plants, focusing on the loquat (Eriobotrya japonica) as a case study. We examine the effectiveness of cut-style pollination in overcoming pre-fertilization barriers and confirm hybridity using molecular markers. This research provides valuable insights for breeding programs aiming to leverage wild genetic resources for crop improvement.
Overcoming Pre-fertilization Barriers in Loquat Hybridization
Reproductive isolation often hinders distant hybridization, manifesting as poor pollen germination, inhibited pollen tube growth, or failure of fertilization. This study investigated the impact of cut-style pollination, a technique involving the removal of a portion of the style, on the success rate of crosses between E. japonica ‘Dawuxing’ and two wild relatives: E. deflexa (winter-blooming) and E. bengalensis (spring-blooming).
Figure 1. Impact of cut-style pollination on pollen germination and tube growth in loquat. Different treatments involved varying timing and methods of style cutting.
Our results demonstrate that the upper third of the style presents a significant barrier to pollen tube growth in incompatible crosses, likely due to callose deposition. Cut-style pollination effectively circumvented this barrier, significantly improving pollen tube growth into the ovary and subsequent fertilization. Specifically, removing the upper third of the style two hours after self-pollination (treatment Ed-2 for E. deflexa pollen and Eb-2 for E. bengalensis pollen) yielded the highest fruit set and seed number compared to other treatments.
Figure 2. Effect of cut-style pollination on reproductive success in loquat. Cut-style pollination enhanced fruit set and seed number, particularly in the Eb-2 and Ed-2 treatments.
Figure 3. Microscopic observation of pollen tube growth in loquat styles. Callose deposition (indicated by arrows) inhibits pollen tube growth in incompatible crosses.
This suggests that self-pollination triggers physiological changes in the style that, when interrupted by style cutting, facilitate the growth of incompatible pollen tubes. This finding contrasts with studies in other plant species where cut-style pollination proved ineffective, highlighting the species-specific nature of reproductive barriers. Our study underscores the importance of optimizing the timing and method of style cutting for successful distant hybridization.
Confirming Hybridity through Molecular Markers
Morphological assessment alone is often insufficient to confirm hybridity in distant crosses. Therefore, we employed allele-specific polymerase chain reaction (AS-PCR) targeting the S-RNase gene, a key determinant of self-incompatibility in Rosaceae. Analysis of 50 random seedlings from both the Eb-2 and Ed-2 crosses revealed S-genotypes consistent with Mendelian inheritance, confirming the hybrid nature of the offspring. All seedlings inherited S-alleles from both parents, confirming successful hybridization.
Implications for Plant Breeding
This comparative study demonstrates that cut-style pollination, when appropriately timed, can effectively overcome pre-fertilization barriers in loquat, enabling successful crosses with wild relatives. The use of molecular markers, such as AS-PCR targeting the S-RNase gene, provides a reliable method for confirming hybridity. These findings have significant implications for loquat breeding programs, allowing for the introgression of valuable traits from wild species, such as disease resistance, stress tolerance, and novel fruit characteristics. This approach could lead to the development of improved loquat cultivars with enhanced productivity and adaptability. Furthermore, this study contributes to a broader understanding of distant hybridization in plants and offers valuable insights for similar efforts in other crops.
