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Sexual Reproduction in the Citrus Black Spot Pathogen, Phyllosticta citricarpa
Nga T Tran1, Andrew K Miles1, Ralf G Dietzgen1
1First, second, and seventh authors: Centre for Plant Science, Queensland Alliance for Agriculture and Food Innovation (QAAFI), The University of Queensland, Ecosciences Precinct, 41 Boggo Road, Dutton Park, QLD 4102, Australia; third author: Centre for Plant Science, QAAFI, The University of Queensland, Queensland Bioscience Precinct, St. Lucia, QLD 4072, Australia; fourth and fifth authors: Citrus Research and Education Centre, University of Florida, Lake Alfred; and sixth author: Department of Plant Pathology, University of Florida, Gainesville.
Abstract:
Citrus black spot (Phyllosticta citricarpa) causes fruit blemishes and premature fruit drop, resulting in significant economic losses in citrus growing areas with summer rainfall across the globe. The mating type locus of P. citricarpa has recently been characterized, revealing the heterothallic nature of this pathogen. However, insight into the occurrence of mating and the impact of completing the sexual cycle of P. citricarpa was lacking. To investigate the occurrence and impact of sexual reproduction, we developed a method to reliably, and for the first time, produce ascospores of P. citricarpa on culture media. To demonstrate meiosis during the mating process, we identified recombinant genotypes through multilocus genotyping of single ascospores. Because the process of fertilization was not well understood, we experimentally determined that fertilization of P. citricarpa occurs via spermatization. Our results demonstrate that P. citricarpa is heterothallic and requires isolates of different MAT idiomorphs to be in direct physical contact, or for spermatia to fulfill their role as male elements to fertilize the receptive organs, in order to initiate the mating process. The impact of mating on the epidemiology of citrus black spot in the field is discussed.