Cytogenomic and transcriptomic approaches to understand nuclear cycle of rusts

Helena Azinheira 1,2 Teresa Ribeiro 1 Rita Carvalho 1 Sílvia Tavares 1,2,3 Marta Monteiro 5 Marco Coelho 4 Maria Silva 1,2 João Loureiro 6 Leonor Morais-Cecílio 1 Pedro Talhinhas 1,2
1Linking Landscape, Environment, Agriculture and Food, Instituto Superior de Agronomia, Universidade de Lisboa, Lisbon, Portugal
2Centro de Investigação das Ferrugens do Cafeeiro, Instituto Superior de Agronomia, Universidade de Lisboa, Oeiras, Portugal
3Department of Plant and Environmental Sciences, Section for Plant and Soil Science, Faculty of Science, University of Copenhagen, Copenhagen, Denmark
4UCIBIO-REQUIMTE, Departamento de Ciências da Vida, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Caparica, Portugal
5IGC, Instituto Gulbenkian de Ciência, Oeiras, Portugal
6Centro de Ecologia Funcional, Departamento de Ciências da Vida, Universidade de Coimbra, Coimbra, Portugal

Fungal life cycles are characterised by predominant haploid stages, while in most cases diploid nuclei occur only in a single cell (the basidium or the ascus, in Basidiomycota and Ascomycota respectively), following karyogamy between haploid nuclei and immediately followed by meiosis. Rust fungi are also reported as obeying to this general rule, with the basidium as the single diploid cell. The haploid cycle in rust fungi is divided in two stages, the first comprising monokaryotic cells (from basidiospores to pycniospore formation), and the second, following plasmogamy, comprising dikaryotic cells (from pycniospore conjugation through aeciosporic and urediniosporic stages to karyogamy occurring in teliospore leading to the diploid basidium). Recently, flow cytometric studies to quantify rusts genome size showed the presence of 1C, 2C and a low proportion of 4C nuclei in different stages of the urediniosporic cycle of several rust fungi, namely Hemileia. vastatrix. These results suggest the presence of diploid nuclei that supposedly only occur in teliospores, and are compatible with the occurrence of karyogamy and meiosis prior to urediniospore formation, although endopolyplody or other parasexuality phenomena cannot be ruled out. In this work we combined cytogenomic techniques with a transcriptomic approach along the infection cycle of H. vastatrix in Coffea arabica leaves to confirm the occurrence of diploid nucleus and the expression of genes related with karyogamy and/or meiosis aiming to enlighten the nuclear cycle of rusts