Abstract
Phytophthora agathidicida is associated with a root rot that threatens the long-term survival of the iconic New Zealand kauri. Although it is widely assumed that this pathogen arrived in New Zealand post-1945, this hypothesis has yet to be formally tested. Here we describe evolutionary analyses aimed at evaluating this and two alternative hypotheses. As a basis for our analyses, we assembled complete mitochondrial genome sequences from 16 accessions representing the geographic range of P. agathidicida as well as those of five other members of Phytophthora clade 5. All 21 mitogenome sequences were very similar, differing little in size with all sharing the same gene content and arrangement. We first examined the temporal origins of genetic diversity using a pair of calibration schemes. Both resulted in similar age estimates; specifically, a mean age of 303.0-304.4 years and 95% HPDs of 206.9-414.6 years for the most recent common ancestor of the included isolates. We then used phylogenetic tree building and network analyses to investigate the geographic distribution of the genetic diversity. Four geographically distinct genetic groups were recognised within P. agathidicida. Taken together the inferred age and geographic distribution of the sampled mitogenome diversity suggests that this pathogen diversified following arrival in New Zealand several hundred to several thousand years ago. This conclusion is consistent with the emergence of kauri dieback disease being a consequence of recent changes in the relationship between the pathogen, host, and environment rather than a post-1945 introduction of the causal pathogen into New Zealand.
Highlights
The genus Phytophthora currently consists of ~180 formally described species and hybrids [1]. While this globally important group of pathogens affects a great many plant species, individual Phytophthora species differ markedly in host range; some have a limited number of hosts (e.g., P. infestans (Mont.) de Bary) while others have upwards of a hundred potential hosts (e.g., P. cinnamomi Rands, P. nicotianae Breda de Haan) [2]
The reported mitochondrial genome sequences can be obtained from Genbank and from the S1 Data
We assembled complete mitochondrial genomes from 21 accessions representing all currently recognised members of Phytophthora clade 5. These mitogenome sequences are highly similar; for example, they differed little in size sharing both gene content and arrangement
Summary
The genus Phytophthora currently consists of ~180 formally described species and hybrids [1]. While this globally important group of pathogens affects a great many plant species, individual Phytophthora species differ markedly in host range; some have a limited number of hosts (e.g., P. infestans (Mont.) de Bary) while others have upwards of a hundred potential hosts (e.g., P. cinnamomi Rands, P. nicotianae Breda de Haan) [2].
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