Megagasporogenesis and megagametogenesis in diploid and tetraploid cytotypes of Paspalum rufum

MARIANO, SOLIMAN; ESPINOZA, FRANCISCO; JUAN PABLO A. ORTIZ; DELGADO LUCIANA

Bahía Blanca

Workshop; V Ciclo de seminarios sobre avances en la caracterización genética y molecular de la apomixis; 2016

CERZOS-CONICET

Paspalum rufum Nees is a robust erect perennial native grass belongingto the Poaceae family and the Panicoideae subfamily. Natural populations form amultiploid complex composed mainly of diploid (2n = 2x = 20) and tetraploid (2n= 4x = 40) cytotypes.  Diploid forms aresexual and highly self-sterile, while tetraploid ones are pseudogamousaposporous apomictic and self-fertile. Previous works revealed that some diploidcytotypes of P. rufum are able to developaposporous embryo sacs. Some individuals are even capable to produce up to 15%of their progenies by apomixis in interspecific and interploidy crosses. Adetailed developmental calendar of tetraploid Paspalum notatum waspreviously constructed, establishing a temporal relationship between pollen andovule development. However there is no specific information about P. rufum reproductive development up todate. The objectives of the present work were: 1) to characterize thereproductive structures of the diploid cytotype of P. rufum in order to use them as a diploid model for apomixisresearch; 2) to compare the reproductive development between diploid sexual andtetraploid apomictic cytotypes. Developmental characterization was done byqualitative observations and quantitative determinations of cleared ovaries andalso of sectioned stained ovaries. All diploid accessions analyzed were sexual,but produced aposporous embryo sacs in low proportions. The cytoembryological analysisallowed us to recognize, at the diploid level, all the reproductive structuresof megasporocytes and megagametophytes, and to establish a useful correlationwith pollen development. Both microsporocyte and megasporocyte development are perfectlycoupled in the diploid cytotype at all analyzed stages. However, in tetraploid cytotypes,cytoembryological observation detected chronological differences, since megasporogenesisis delayed with respect to microsporogenesis. In accordance with theseobservations, meiosis also seems to be delayed in tetraploid respect to diploidcytotype, regarding integuments? growth. The analysis of aposporous developmentin diploids shows that aposporous initials (AI) appear while meiosis isoccurring. Contrastingly, in tetraploids AIs arise when the megaspore mothercells (MMCs) differentiate. Besides, aposporous embryo sac development in the tetraploidcytotypes is advanced in relation to sexual embryo sacs development. Our resultscontributed enough information to construct reproductive calendars of P. rufum diploid and tetraploidcytotypes and also to support the existence of differences in the reproductive developmentaltiming between diploid and tetraploid genotypes. According to our results, thefemale meiosis is delayed in tetraploid genotypes.