Parthenocarpy and Self-Incompatibility in Mandarins

Abstract

Citrus reproductive biology is complex. One of its characteristic features is parthenocarpy that enables seedless fruit production. Citrus parthenocarpy and self-incompatibility knowledge is only partial and sometimes discrepant. Increasing such knowledge is relevant for better managing cultivated varieties and improving the selection of parents in breeding strategies to recover seedless varieties such as mandarins. This work develops an efficient protocol to characterize self-incompatibility and different parthenocarpy types based on emasculation, hand self-pollination, and hand cross-pollination. It analyzes fruit setting and seed production coupled with histological pollen performance observations. We analyzed the reproductive behavior of nine mandarin varieties with relevant characteristics as parents for seedless mandarin breeding. ‘Clemenules’ clementine and ‘Moncada’ mandarins were strictly self-incompatible with facultative and vegetative parthenocarpy; ‘Imperial’ mandarin and ‘Ellendale’ tangor displayed no strict self-incompatibility associated with facultative and vegetative parthenocarpy; ‘Fortune’ mandarin was self-incompatible with facultative and stimulative parthenocarpy; ‘Campeona’ and ‘Salteñita’ mandarins were self-compatible with vegetative parthenocarpy; ‘Serafines’ satsuma was associated with male sterility together with facultative and vegetative parthenocarpy; and ‘Monreal’ clementine was self-compatible and nonparthenocarpic. Our protocol can be applied for screening of mandarin germplasm and to characterize new parents. Reproductive behavior knowledge is important for optimizing seedless mandarin breeding programs based on diploidy, triploidy, or induced mutagenesis.

Programmed Cell Death May Be Involved in the Seedless Phenotype Formation of Oil Palm

“…Self-incompatibility, also known as self-sterility, is a mechanism that rejects their self-pollen and inhibits seed formation in the absence of cross-pollination ( Montalt et al, 2021 ). Molecular studies revealed that genes within the S-locus, including S-RNase, SRK, and SLG, regulate the SI system ( Kachroo et al, 2002 ; Wang et al, 2009 ; Liang et al, 2020 ).…”

“…The over-expression of OsAAE3 in rice results in programmed cell death (PCD) and, consequently decreases pollen fertility ( Liu et al, 2017a ). Seedlessness in citrus has been reported to be associated with ovule sterility or self-incompatibility (SI; Montalt et al, 2021 ). SI system prevents self-fertilization by rejecting self-pollen in the style of flowering plants ( Claessen et al, 2019 ).…”

Breakdown of Self-Incompatibility in Citrus by Temperature Stress, Bud Pollination and Polyploidization

“…Physiological changes may change over time and can induce different responses to the SI reaction in some genotypes. For example, in a previous work [46], we classified 'Imperial' mandarin and 'Ellendale' tangor as non-strict self-incompatible genotypes because they produced both seedless and low-seeded fruit from self-pollinated flowers, which suggests an impact of environmental conditions [90]. Similarly, Claessen et al [12] indicated in pear and apple that the strength of the SI reaction differed depending on distinct intrinsic and extrinsic factors, including flower age and quality, temperature and application of plant hormones.…”

“…Several mechanisms have been described in citrus that, when coupled with parthenocarpy, produce seedless fruit; these include for example, ovule sterility in flowers with no functional pistils [35], degeneration [36] and chromosome aberrations in the embryo sac [37], stenospermocarpy [38,39], gene-cytoplasmic interaction [40], gamma irradiation [41,42] and triploidy [43][44][45]. Besides all these mechanisms, SI is also an efficient way to produce seedless genotypes [43,46]. The most important self-incompatible horticultural citrus groups are pummelos (Citrus maxima (Burm.)…”

“…By taking advantage of histological techniques to observe pollen tube growth inside pistils, different authors have demonstrated that the SI reaction takes place between the stigma and the ovary [47,49]. For mandarin hybrids, including the 'Fortune' mandarin (C. clementina x C. tangerina), and the 'Clemenules' clementine, several authors agree that the SI reaction originates in the upper part of the style [46,47,[50][51][52].…”