APOMIXIS IN Eragrostis curvula

ECHENIQUE VIVIANA

Rosario

Congreso; XV Congreso Latinoamericano de Genética, XLI Congreso Argentino de Genética, XLV Congreso de la Sociedad de Genética de Chile y II Reunión Regional SAG-Litoral.; 2012

Sociedad Argentina de Genética

Over the past few years our research group has analyzed the molecular mechanisms involved in the reproductive control of weeping lovegrass (Eragrostis curvula [Schrad.] Nees), an apomictic perennial grass originally native to Tanzania. The E. curvula complex includes cytotypes with different ploidy levels (from 2x to 8x) that may undergo sexual reproduction, facultative apomixis or obligate apomixis. Diploid (2n = 2x = 20) plants are sexual, rare, and do not occur in all forms of E. curvula. Polyploids reproduce mainly by obligate apomixis, but facultative apomixis has also been reported. The type of apomixis present in this grass is pseudogamous diplospory. Meiotic stages are absent from the Eragrostis-type apomixis, and the megasporocyte undergoes only two rounds of mitotic divisions to form a non-reduced tetranucleated embryo sac with an egg, two synergids, and one polar nucleus. The establishment of an E. curvula euploid ?back-and-forth? (4x-2x-4x) series, displaying different ploidy levels and reproductive modes but sharing a common genetic background was useful to study the reproductive mode of this grass. We analyzed the genome structucture and the transcriptome profiles of these plants and also the methylation level using the MSAP technique. From a total of 8,884 unigenes sequenced from inflorescence-derived libraries, 112 (1.26%) showed significant differential expression in individuals with different ploidy level and/or variable reproductive mode. Differentially expressed genes were mapped in silico onto maize chromosomes, with detailed focus on the linkage group syntenic to the reported Tripsacum dactyloides diplospory-governing region. Evidence indicates that expression of genes located around the diplospory region might be strongly influenced by ploidy and silenced in the apomictic genotype. The change in apomixis expression level observed in these colchiploids between 2003 and 2007, from vestigial to almost 90%, prompted several relevant questions. The objective of this study was to analyze the molecular basis of apomixis variable expressivity and its link to extrinsic and intrinsic stress. We examined genetic/epigenetic modifications affecting the genome of an Eragrostis curvula artificial polyploid (C) whose reproductive behavior changed from highly sexual to apomictic during 2003-2007. Results indicated that, during this interval, genetic and epigenetic structures of both plant C and an obligate apomictic control T were modified. While genetic variation led to diversification, epigenetic variation resulted in increased similarity. Methylation processes were prevalent in C compared with control T. Many of the sequences that were genetically or epigenetically modified during the 2003-2007 period were similar to transposable elements (TEs) and/or pseudogenes. EST analysis revealed more TE transcripts present in inflorescences than in leaves, suggesting a role in reproductive development. Furthermore, the gypsy retroelement subfamily was more highly represented in inflorescences of colchiploid C compared with the natural apomict T and a dihaploid sexual plant D. Eragrostis curvula plants subjected to both water and in vitro culture stress showed an increase in the rate of sexuality. Our results suggest that in the presence of stress, sexuality can be temporarily activated in apomictic genotypes, and that retroelement activity is involved in the regulation of the switch from sexuality to apomixis.