Abstract
The adaptive rapid evolution of phenotypic traits is potentially a key contributor to invasiveness, but has been relatively little studied for the fungi, even though these organisms are responsible for devastating losses in agriculture and natural resources. In this study, we compare biologically relevant phenotypic characters of spore‐generated individuals from two native and two invasive populations of the fungal pathogen Seiridium cardinale to infer which traits may be adaptive and rapidly evolving during an ongoing biological invasion. Results show that: (1) lower growth rate and smaller spore size are selected for in invasive populations, independent of the stage of invasion; (2) there is no selection evident towards increased rapid sporulation, but overall reproductive potential increases in later stages of the invasions; and (3) demographic plasticity of most traits increases during the initial stages of invasion, but decreases in a later phase. Comparisons against levels of neutral genetic variation (Qst‐Fst comparisons) showed that the decrease in spore size is strongly adaptive, despite the trade‐off of reduced viability. Lesion size of isolates inoculated on the naïve Italian cypress host was not correlated with their growth rate, and was significantly lower in invasive than in native populations. This last result indicates that rate of host colonization is a complex trait affected both by host and pathogen, which may not be necessarily adaptive and/or which may not easily evolve. In summary, the success of S. cardinale as an invasive in the Mediterranean basin is associated with reduced spore size and increased plasticity of almost all traits in initial phases, followed by further decreased spore size, increased overall sporulation, and decreased plasticity in a second phase of the invasion. Interestingly, growth rate by population results show that invasive populations are well adapted only to moderate temperatures, while native populations fare well also when exposed to relative extremes in temperature. This different adaptation suggests a “master‐of‐some” specialization scenario for the invasion by S. cardinale in the Mediterranean.
Highlights
As they expand their ranges and encounter new environments, invasive species provide an invaluable opportunity to understand the evolution of adaptation in organisms in real-time (Suarez and Tsutui 2008)
Because invasions are often initiated by the long-distance movement of a few individuals, the extent of phenotypic variability in the presence of limited genetic variability may provide insights into the role plasticity may play in the invasion process, a concept that has been often theoretically suggested, but little studied (Richards et al 2006)
This is true for clonally reproducing organisms, in which the genetic make up of founder individuals has the potential to influence invasive populations for periods of considerable length, as mutation-derived adaptation occurs at much lower frequency than that driven by recombination in sexually reproducing species (Fisher 1930, Muller 1932)
Summary
As they expand their ranges and encounter new environments, invasive species provide an invaluable opportunity to understand the evolution of adaptation in organisms in real-time (Suarez and Tsutui 2008). Because invasions are often initiated by the long-distance movement of a few individuals, the extent of phenotypic variability in the presence of limited genetic variability may provide insights into the role plasticity may play in the invasion process, a concept that has been often theoretically suggested, but little studied (Richards et al 2006) This is true for clonally reproducing organisms, in which the genetic make up of founder individuals has the potential to influence invasive populations for periods of considerable length, as mutation-derived adaptation occurs at much lower frequency than that driven by recombination in sexually reproducing species (Fisher 1930, Muller 1932). The inclusion of the correct source population is critical for studying trait variation of an invasive population through comparison with native populations (Keller and Taylor 2008)
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