Detalle de: Anomalous embryo sac development and fruit abortion caused by inbreeding depression in almond (Prunus dulcis)

The decrease in fitness of the inbred offspring in relation to the outbred offspring is termed inbreeding depression, and is attributed to the expression of deleterious recessive alleles. This may represent an important problem for future almond growing, since the use of a very limited germplasm base as source of self-compatibility in breeding programmes has presumably led to a considerable increase of the inbreeding degree in subsequent progenies, from which new cultivars will be released. In the present study, the effects of inbreeding depression on flower morphology, pollen viability, pollen tube growth and fruit set were evaluated after self- and cross-pollination of two self-compatible almond cultivars and 12 descendants obtained from successive self-fertilizations. Moreover, in those descendants with very low or null fruit set values and in the cultivar ‘Tuono’ (first generation), the embryo sac development was studied. The self-compatible cultivar ‘Antoneta’ was included as external control in the experimental assays. In general, there were no differences between both types of pollination, and only those individuals with higher inbreeding levels were affected by inbreeding depression, which was manifested by lower pollen germination, decelerated growth of pollen tubes through the style, delayed ovule development at the prezygotic phase, and high fruit drop. This high fruit drop seemed to be due to the presence of many aborted fruits with an anomalous postzygotic development of the embryo sac, finally resulting in necrosis at the chalazal end and lack of endosperm cellularization. Therefore, data presented here indicate that the low or null fruit set observed after fertilization in highly inbred almond genotypes seems to be a consequence of inbreeding depression, and highlight the importance of avoiding successive self-fertilizations as strategy of breeding in this species.