Isolation and molecular characterization of Fusarium species (Fungi, Ascomycota) from unhatched eggs of Caretta caretta in Tuscany (Italy)

Abstract

Fungal infectious diseases have dramatically increased in marine ecosystems during the past two decades and actually represent one of the main threats to biodiversity, likely due to the occurrence of emerging pathogens in new environments and the stress conditions induced by global climate change. In this context, the loggerhead sea turtle (Caretta caretta L.) is a vulnerable species according to the International Union for Conservation of Nature (IUCN) and it is included as a protected species under several international conventions. Sea Turtle Egg Fusariosis (STEF) is a worldwide emergent fungal disease associated with egg and embryos mortality in endangered sea turtle nests such as those of C. caretta. The disease can lead to a significant mass mortality in the infected nests and is caused by a complex of species belonging to Fusarium genus with isolates included in the Fusarium Solani Species Complex (FSSC); however, many questions regarding the aetiology and epidemiology of this disease as well as the biology and ecology of the causal agents are still open. C. caretta is the only sea turtle species nesting along the Tuscan archipelago where nests are becoming more numerous and widespread. At the same time, in the recent years a continuous monitoring of nesting and hatching sites allowed to record an increased number of affected nests, probably due to STEF. During the monitoring activities conducted in 2019-2020 in several localities on the Tuscany coast (province of Grosseto), a large number of eggs showing symptoms resembling those caused by STEF were found. Symptomatic eggs were so collected from nests located in three beaches and a total of 32 fungal isolates were obtained and submitted to a morphological identification followed by a molecular characterization. Amplicons were sequenced and used to assign the species, thus allowing to identify our isolates as Fusarium solani, Fusarium oxysporum and Fusarium nodosum. Finally, the phylogenetic relationships between our strains and those already known was rebuilt. While F. solani and F. oxysporum were already associated with C. caretta eggs showing symptoms of fungal infection, as far as we know, this is the first time that F. nodosum was isolated from affected eggs. Furthermore, this work represents the first report of STEF on Tuscan coast. Although Tuscany does not represent a primary nesting area of C. caretta in the Mediterranean basin, the record of the disease on this coastline, in line with what is happening across the globe, confirms that STEF may represent a major risk for the conservation of the loggerhead sea turtle also in this region.