Self-pollen interference is absent in wild radish (Raphanus raphanistrum, Brassicaceae), a species with sporophytic self-incompatibility

Fine mapping of a major co-localized QTL associated with self-incompatibility identified in two F2 populations (broccoli × cauliflower and cauliflower × Chinese kale)

KEY MESSAGE

A major QTL responsible for self-incompatibility was stably identified in two F2 populations. Through fine mapping and qRT-PCR analysis, ARK3 emerged as the most promising candidate gene, playing a pivotal role in regulating self-incompatibility in Brassica oleracea. Self-incompatibility (SI) is a common phenomenon in Brassica oleracea species, which can maintain genetic diversity but will also limit seed production. Although the S locus has been extensively studied in Arabidopsis and some Brassicaceae crops, map-based cloning of self-incompatibility genes has not been conducted in Brassica oleracea, such as cauliflower and broccoli. In the present study, we identified a major co-localized QTL on chromosome C6 that control SI in two F2 populations derived from intervarietal crosses: broccoli × cauliflower (CL_F2) and cauliflower × Chinese kale (CJ_F2). Subsequently, this QTL was narrowed down to 168.5 Kb through fine mapping using 3,429 F2:3 progenies and 12 available KASP markers. Within this 168.5 Kb region, BolC6t39084H, a homologue of Arabidopsis ARK3, could be a candidate gene that plays a key role in regulating SI in B. oleracea species. This finding can pave the way for an in-depth understanding of the molecular mechanisms underlying SI, and will contribute to the seed production of B. oleracea vegetables.

Harmful self-pollination drives gynodioecy in European chestnut, a self-incompatible tree

PREMISE

Gynodioecy is a rare sexual system in which two genders (sensu Lloyd, 1980), cosexuals and females, coexist. To survive, female plants must compensate for their lack of siring capacity and male attractiveness. In European chestnut (Castanea sativa), an outcrossing tree, self-pollination reduces fruit set in cosexual individuals because of late-acting self-incompatibility and early inbreeding depression. Could this negative sexual interaction explain the presence of females in this species?

METHODS

We studied gender variation in wild populations of European chestnut. In addition, we compared fruit set (the proportion of flowers giving fruits) and other key female fitness components as well as reproductive allocation between genders. We then performed emasculation experiments in cosexual trees, by removing nectar-producing fertile male inflorescences. We also removed sterile but nectar-producing male inflorescences from female trees, as a control.

RESULTS

We found a highly variable proportion of male-sterile individuals in the wild in European chestnut. In the experimental plot, trees from each gender had similar size, flower density, and burr set, but different fruit set. Removing nectar-producing male inflorescences from branches or entire trees increased fruit set in cosexual but not in female trees.

CONCLUSIONS

These results show that self-pollination impairs fruit set in cosexual trees. Female trees avoid these problems as they do not produce pollen but continue to attract pollinators thanks to their rewarding male-sterile inflorescences, resulting in a much higher fruit set than in cosexuals. This demonstrates that even outcrossed plants can benefit from the cessation of self-pollination, to the point that unisexuality can evolve.

The influence of self-pollen deposition on female reproductive success in a self-incompatible plant, Akebia quinata

Geitonogamy is inevitable in hermaphrodite and monecious. Even for self-incompatible species, the negative effects of self-pollen are unavoidable when geitonogamous or self-mating occurs. However, the influence of self-pollen on consecutive development of flowers (e.g., fruiting and seeding) was seldom evaluated. Here, the self-incompatible monecious species, Akebia quinata, was used to estimate the influence of self-pollen deposition. We evaluated the extent of pollen limitation and geitonogamous mating under natural conditions by count of stigmatic pollen load and pollen tracking experiment. Hand pollination with different amount and combinations of self vs. cross pollen grains was applied to detect the response of fruit and seed set. The results showed that geitonogamy and pollen limitation occurred under natural conditions in A. quinata. Carpel numbers, ratio of self- and cross-pollen, and the interactive effect of ratio of self- and cross-pollen and total mixed pollen numbers, and not total pollen grain number, determined the effect of self-pollen on female reproductive success. The effect of self-pollen depended on its intensity. In general, the transfer of self-pollen significantly affected young fruit set. However, a little self-pollen together with cross-pollen did not reduce young fruit production. Although self-incompatible plants have evolved physiological mechanisms that reduce self-fertilization, our results provide new insights into the effects of self-pollen and the adaptive significance of self-incompatible monecious species.

Inheritance of 2,4-D resistance traits in multiple herbicide- resistant Raphanus raphanistrum populations

A relatively low number of weed species have evolved resistance to auxinic herbicides despite their use for almost 70 years. This inheritance study with two Raphanus raphanistrum populations multiple-resistant 2,4-D and the ALS-inhibiting herbicide chlorsulfuron determined the number of genes and genetic dominance of 2,4-D resistance and investigated the association between traits conferring resistance to the two herbicide modes of action. Levels of 2,4-D phenotypic resistance and resistance segregation patterns were assessed in parental populations, F₁ and F₂ families.

Late-acting self-incompatibility--the pariah breeding system in flowering plants

It is estimated that around half of all species of flowering plants show self-incompatibility (SI). However, the great majority of species alleged to have SI simply comply with 'the inability of a fully fertile hermaphrodite plant to produce zygotes when self-pollinated'--a definition that is neutral as to cause. Surprisingly few species have been investigated experimentally to determine whether their SI has the type of genetic control found in one of the three established mechanisms, that is, homomorphic gametophytic, homomorphic sporophytic or heteromorphic SI. Furthermore, our knowledge of the molecular basis of homomorphic SI derives from a few species in just five families--a small sample that has nevertheless revealed the existence of three different molecular mechanisms. Importantly, a sizeable cohort of species are self-sterile despite the fact that self-pollen tubes reach the ovary and in most cases penetrate ovules, a phenomenon called late-acting self-incompatibility (LSI). This review draws attention to the confusion between species that show 'self-incompatibility' and those that possess one of the 'conventional SI mechanisms' and to argue the case for recognition of LSI as having a widespread occurrence and as a mechanism that inhibits selfing and promotes outbreeding in many plant species.