Backcrossing Failure between Sikitita Olive and Its Male Parent Arbequina: Implications for the Self-Incompatibility System and Pollination Designs of Olive Orchards

Abstract

Backcrossing between Sikitita and its male parent Arbequina, offers the possibility to check the suitability of different self-incompatibility models proposed for olive. To determine Sikitita's response to self- and cross-pollination treatments, including pollination with its father Arbequina, we compared the parameters following pollen-pistil interaction, the resulting initial and final fruit set, and the paternity of the seeds produced under different crosses. The results showed that Sikitita behaves as a self-incompatible cultivar due to the inhibition of pollen tube growth in the pistil of self-pollinated flowers. This incompatibility reaction led to a significant reduction of self-fertilization and fruit set. Seed paternity analyses confirmed the self-incompatibility response of Sikitita. A similar incompatibility response was observed in Sikitita flowers when hand-pollinated with pollen of Arbequina and Koroneiki. On the contrary, cross-pollination with Arbosana gave excellent results, with analyses showing that pollen of Arbosana is largely preferred by Sikitita to father its seeds more than the pollen of other cultivars presented in the orchard. The backcross failure of Sikitita with Arbequina pollen suggests that the self-incompatibility system in olives is not of the gametophytic type. In contrast, pollination tests fit features of previously reported sporophytic self-incompatibility systems. However, some amendments are proposed, among them the incompatibility groups for Sikitita and Koroneiki.

Keywords: Olea europaea; fruit set; gametophytic self-incompatibility; pollen–pistil interactions; pollination groups; pollinizer selection; seed paternity; sporophytic self-incompatibility.

Figure 1

Gametophytic (GSI) and sporophytic self-incompatibility (SSI) models as proposed by Breton and Bervillé in [16]. Published by Elsevier Masson SAS. All rights reserved. X means pollen rejection; the arrow means pollen acceptance; > means dominant upon; 00 means no possible progenies.

Figure 2

Singular and hypothetical S-genotypes for Sikitita and its parents (Picual as pollen recipient × Arbequina as pollen donor) that could only explain the backcross failure between Sikitita and its father Arbequina in a gametophytic self-incompatibility system.

Figure 3

Temperature (maximum, minimum and average, in red, blue and black lines, respectively) average humidity (green line), and rainfall (yellow bars) during the experimental seasons 2022 (a) and 2023 (b). Data retrieved from a weather station sited in the experimental area at the Campus of the University of Córdoba.

Figure 4

Scale from 0 to 3 to quantify pollen tube growth based on the number traversing the stigma and style. Published by Sánchez-Estrada and Cuevas [55]. With Permission of HortTechnology, a Journal of the American Society for Horticultural Science. Before squashing, the length of the stigma was in the range of 1–1.3 mm, and width of stigma between 0.7 and 0.9 mm.