Evolution of Transcriptomes in Early-Generation Hybrids of the Apomictic Ranunculus auricomus Complex (Ranunculaceae).

Hybridisation in plants may cause a shift from sexual to asexual seed formation (apomixis). Indeed, natural apomictic plants are usually hybrids, but it is still unclear how hybridisation could trigger the shift to apomixis. The genome evolution of older apomictic lineages is influenced by diverse processes such as polyploidy, mutation accumulation, and allelic sequence divergence. To disentangle the effects of hybridisation from these other factors, we analysed the transcriptomes of flowering buds from artificially produced, diploid F2 hybrids of the Ranunculus auricomus complex. The hybrids exhibited unreduced embryo sac formation (apospory) as one important component of apomixis, whereas their parental species were sexual. We revealed 2915 annotated single-copy genes that were mostly under purifying selection according to dN/dS ratios. However, pairwise comparisons revealed, after rigorous filtering, 79 genes under diversifying selection between hybrids and parents, whereby gene annotation assigned ten of them to reproductive processes. Four genes belong to the meiosis-sporogenesis phase (ASY1, APC1, MSP1, and XRI1) and represent, according to literature records, candidate genes for apospory. We conclude that hybridisation could combine novel (or existing) mutations in key developmental genes in certain hybrid lineages, and establish (together with altered gene expression profiles, as observed in other studies) a heritable regulatory mechanism for aposporous development.

The relation of meiotic behaviour to hybridity, polyploidy and apomixis in the Ranunculus auricomus complex (Ranunculaceae).

BACKGROUND: Hybridization and polyploidization are powerful evolutionary factors that are associated with manifold developmental changes in plants such as irregular progression of meiosis and sporogenesis. The emergence of apomixis, which is asexual reproduction via seeds, is supposed to be connected to these factors and was often regarded as an escape from hybrid sterility. However, the functional trigger of apomixis is still unclear. Recently formed di- and polyploid Ranunculus hybrids, as well as their parental species were analysed for their modes of mega- and microsporogenesis by microscopy. Chromosomal configurations during male meiosis were screened for abnormalities. Meiotic and developmental abnormalities were documented qualitatively and collected quantitatively for statistical evaluations. RESULTS: Allopolyploids showed significantly higher frequencies of erroneous microsporogenesis than homoploid hybrid plants. Among diploids, F2 hybrids had significantly more disturbed meiosis than F1 hybrids and parental plants. Chromosomal aberrations included laggard chromosomes, chromatin bridges and disoriented spindle activities. Failure of megasporogenesis appeared to be much more frequent in than of microsporogenesis is correlated to apomixis onset. CONCLUSIONS: Results suggest diverging selective pressures on female and male sporogenesis, with only minor effects of hybridity on microsporogenesis, but fatal effects on the course of megasporogenesis. Hence, pollen development continues without major alterations, while selection will favour apomixis as alternative to the female meiotic pathway. Relation of investigated errors of megasporogenesis with the observed occurrence of apospory in Ranunculus hybrids identifies disturbed female meiosis as potential elicitor of apomixis in order to rescue these plants from hybrid sterility. Male meiotic disturbance appears to be stronger in neopolyploids than in homoploid hybrids, while disturbances of megasporogenesis were not ploidy-dependent.

Establishment of Apomixis in Diploid F2 Hybrids and Inheritance of Apospory From F1 to F2 Hybrids of the Ranunculus auricomus Complex.

Hybridization and polyploidization play important roles in plant evolution but it is still not fully clarified how these evolutionary forces contribute to the establishment of apomicts. Apomixis, the asexual reproduction via seed formation, comprises several essential alterations in development compared to the sexual pathway. Furthermore, most natural apomicts were found to be polyploids and/or hybrids. The Ranunculus auricomus complex comprises diploid sexual and polyploid apomictic species and represents an excellent model system to gain knowledge on origin and evolution of apomixis in natural plant populations. In this study, the second generation of synthetically produced homoploid (2x) and heteroploid (3x) hybrids derived from sexual R. auricomus species was analyzed for aposporous initial cell formation by DIC microscopy. Complete manifestation of apomixis was determined by measuring single mature seeds by flow cytometric seed screen. Microscopic analysis of the female gametophyte formation indicated spontaneous occurrence of aposporous initial cells and several developmental irregularities. The frequency of apospory was found to depend on dosage effects since a significant increase in apospory was observed, when both F1 parents, rather than just one, were aposporous. Other than in the F1 generation, diploid Ranunculus F2 hybrids formed BIII seeds and fully apomictic seeds. The results indicate that hybridization rather than polyploidization seems to be the functional activator of apomictic reproduction in the synthetic Ranunculus hybrids. In turn, at least two hybrid generations are required to establish apomictic seed formation.