Understanding Apple Pollination and Compatibility

Jenn Hoskins
30th April, 2024

Understanding Apple Pollination and Compatibility

Image Source: Natural Science News, 2024

Key Findings

  • Tunisian apple study found common genetic variants influencing self-fertilization prevention
  • Pollination experiments showed varying pollen tube growth, with one variety possibly self-compatible
  • Genetic analysis suggests Tunisian apples may have a modern origin, challenging previous beliefs
Understanding the reproductive mechanisms of plants is crucial for agriculture, especially in fruit crops where self-incompatibility (SI) can limit productivity. SI is a plant's ability to prevent self-fertilization and promote genetic diversity. This mechanism is particularly interesting in apples (Malus domestica. Borkh), which exhibit an S-RNase-based gametophytic self-incompatibility (GSI) system. Recent research from the University of Tunis El Manar[1] has delved into the genetic underpinnings of SI in Tunisian apple varieties, revealing insights that could benefit crop improvement and breeding programs. The study focused on the S-genotypes of 18 Tunisian local apple accessions and compared them with 12 introduced accessions. S-genotypes determine the SI response, with different combinations of S-alleles (the genetic variants at the S-locus) influencing a plant's ability to reject its own pollen. Researchers used four primer combinations to identify the S-alleles present in these accessions. They found that S2, S3, S7, and S28 were the most frequent S-alleles among the Tunisian apples, which were assigned to 14 different S-genotypes. One of the intriguing outcomes was the population structure suggested by the principal component analysis (PCA), which indicated that the Tunisian varieties likely had a modern origin. However, this was put into question by the separate grouping observed when 17 SSR (simple sequence repeat) markers were used. SSR markers are DNA sequences that are repeated in the genome; they are often used to assess genetic diversity and relatedness among individuals. Pollination experiments provided further insights. Researchers observed the germination of pollen and the growth of pollen tubes, which are necessary for fertilization. In the "El Fessi" accession, pollen germination started within 24 hours after pollination, but the pollen tubes took longer to reach the style. Early flowering accessions "Arbi" and "Bokri" had pollen tubes arrive in the styles within 36 hours, while it took 72 hours for the late flowering "El Fessi." Arrests in pollen tube growth, which indicate a functioning GSI system preventing self-fertilization, were observed at different times across the accessions. The exception was the "Boutabgaya" accession, where pollen tubes reached the base of the style without being aborted, suggesting a malfunction in the GSI system. This could indicate that "Boutabgaya" is self-compatible, a trait that is rare in apples and could be advantageous for breeding. However, the study calls for more research to confirm this self-compatible nature. This research ties into previous studies on Prunus species, which include other economically important fruit trees like plums, peaches, and almonds. For instance, the extensive study of S-genotypes in plums[2] has shown the value of such genetic analyses for understanding incompatibility groups and genetic diversity. Similarly, the transition from SI to self-compatibility (SC) in peaches[3] highlights the evolutionary shifts that can occur within the Rosaceae family, to which both plums and apples belong. Moreover, the independent evolution of the GSI system in Malus (apple) compared to Prunus[4] underscores the complexity and diversity of reproductive strategies within closely related species. The findings from the University of Tunis El Manar contribute to a growing body of knowledge about the genetic mechanisms governing SI in fruit trees. By identifying the S-alleles present in Tunisian apples and observing the correlation between flowering periods and pollen tube growth, this study not only provides a foundation for further research into the malfunctioning GSI in "Boutabgaya" but also prompts a reevaluation of the origins of Tunisian apple varieties. The implications for crop improvement are significant, as understanding and potentially manipulating SI can lead to more reliable fruit production.

FruitsGeneticsPlant Science

References

Main Study

1) Self-(in)compatibility in Tunisian apple accessions [Malus domestica. Borkh]: S-genotypes identification and pollen tube growth analysis.

Published 29th April, 2024

https://doi.org/10.1007/s00425-024-04418-x

Related Studies

2) Analysis of Self-Incompatibility and Genetic Diversity in Diploid and Hexaploid Plum Genotypes.

https://doi.org/10.3389/fpls.2019.00896

3) Self-compatibility in peach [Prunus persica (L.) Batsch]: patterns of diversity surrounding the S-locus and analysis of SFB alleles.

https://doi.org/10.1038/s41438-020-00392-z

4) Convergent evolution at the gametophytic self-incompatibility system in Malus and Prunus.

https://doi.org/10.1371/journal.pone.0126138


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