Genomic stress and expression of apomixis in weeping lovegrass

Abstract

Over the past few years our research group has been analyzing the molecular mechanisms involved in the reproductive control of weeping lovegrass (Eragrostis curvula [Schrad.] Nees). The aim of this work was to study the effect of different factors that generate genomic stress on the expression of apomixis. For this purpose, embryo sacs of different weeping lovegrass genotypes were analyzed after the plants were subjected to water stress conditions. Plants of cv. Tanganyika INTA (Ti) subjected to water stress and control plants showed an average of 14% or 2% sexual embryo sacs, indicating an increment of sexuality under stress. After a rehydration period these plants showed an average of sexuality rate of 5.3 %, indicating a significant reduction in the number of sexual embryo sacs, being, however, significantly different from control plants. Plants of the cv. Tanganyika USDA (Tu) did not show at any time the ability to generate sexual embryo sacs and were classified as full apomictic. ABA foliar application was performed on normally watered Ti plants, but it was not possible to detect significant differences in the number of sexual processes between control and treated plants. The effect of in vitro culture was also analyzed taken into account the explant type, culture media and time under culture. In one of the trials a significant effect on the number of sexual embryo sacs was observed, with an increment of 33%. These results indicate that stress situations before and during the flowering stage might be regulating the expression of sexuality in facultative apomictic genotypes. The analysis of the expression of apomixis (number of sexual processes) over time in two hybrids (#105 and #60) obtained from crosses between sexual x apomictic materials showed differences in the expression of the trait. Hybrid #105 showed an erratic behavior in the ratio of apomictic/sexual processes, ending after two years with a ratio of 1/1, while hybrid #60 showed sexual embryo sacs only during the two first blooms and a small proportion of apomictic embryo sacs during the third one. Given the close relationship between apomixis and environment, it might be speculated that the genes involved in the control of the reproductive mode could be subjected to epigenetic regulation. Comparative analyses at genetic (AFLP) and epigenetic (MSAP) levels were performed. The groups compared were: 1) plants under water stress and control belonging to Ti and 2) hybrids #105 and #60. In the first case, no genetic changes were observed while comparisons with MSAP showed a large number of polymorphic markers, showing a high correlation level (R2 = 0.79) between the percentages of changes over time and the number of sexual embryo sacs. Genetic changes were detected neither in hybrid #60 nor in #105. However, the analysis of MSAP profiles showed a higher proportion of methylation processes in hybrid #60 with respect to #105, while DNA de-methylation levels showed an opposite trend. The sequencing of a sample of the polymorphic MSAP bands showed a high proportion (55 %) of sequences without homology to previously annotated sequences, being probably noncoding regions or unknown genes. The other sequences were mainly transposable elements.