Sexual Reproduction in the Citrus Black Spot Pathogen,Phyllosticta citricarpa

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.

Phyllosticta citricarpa and sister species of global importance to Citrus

“…citricarpa was known (Wang et al ., ), it was only recently successfully induced in vitro by Tran et al . () through outcrossing of isolates carrying complementary MAT idiomorphs, thus demonstrating the heterothallic behaviour of P . citricarpa .…”

“…In the same study, the sexual function of spermatia as male gametes in the life cycle of P . citricarpa , and the fertilization in this fungus occurring via spermatization, were also demonstrated (Tran et al ., ). However, no research has to date focused on the sexual morph of the other Phyllosticta spp.…”

“…In Australia, Brazil, China and South Africa, MAT genotyping of populations revealed an almost equal mating type distribution, confirming the occurrence of sexual reproduction in these populations (Amorim et al ., ; Carstens et al ., ; Tran et al ., ; Wang et al ., ; Zhang et al ., ). Interestingly, in Florida in the USA, only one mating type has been identified (Carstens et al ., ; Wang et al ., ).…”

“…The first successful and reproducible mating of P. citricarpa in culture was reported by Tran et al . (), involving direct physical contact between two isolates of complementary mating types. The spermatia function as male gametes and their fertilization of receptive organs during sexual reproduction of P. citricarpa via spermatization was demonstrated.…”

“…Amorim et al (2017) conducted in vitro crossings with isolates of different mating types on various media and incubated plates at different conditions, but did not observe any sexual structures. The first successful and reproducible mating of P. citricarpa in culture was reported by Tran et al (2017), involving direct physical contact between two isolates of complementary mating types. The spermatia function as male gametes and their fertilization of receptive organs during sexual reproduction of P. citricarpa via spermatization was demonstrated.…”