Genetic structure of a population of Ganoderma boninense on oil palm

Abstract

Ganoderma boninense (the causal agent of basal stem rot of oil palm in Papua New Guinea) has a tetrapolar mating system with multiple alleles. Investigations into the population structure of G. boninense , using interfertility between isolates as a marker, revealed that the population on oil palm was comprised predominantly of genetically distinct individuals, although a number of isolates were found to share single mating alleles. No direct hereditary relationship was found between isolates on neighbouring or spatially separated diseased palms, indicating that outcrossing had probably occurred over several generations in the founder population prior to colonization of oil palm. In this study, a total of 81 A and 83 B mating type alleles (factors) were detected with 18 allelic repeats at the A locus and 17 at the B locus. Alleles appeared to be randomly dispersed throughout the population in each study block, although there was a significantly ( P < 0·01) higher proportion of allelic repeats between estates separated by several kilometres compared to within estates. This is the first study on the population structure of this species and confirms that sexual reproduction is a significant component of the epidemiology of this disease.

About Ganoderma boninense in oil palm plantations of Sumatra and peninsular Malaysia: Ancient population expansion, extensive gene flow and large scale dispersion ability

“…In comparison, the white rot fungus Armillaria mellea, (order Agaricales) has a genetic diversity ranging between 0.103 and 0.600, depending on the population considered (Prospero et al 2008;Travadon et al 2012 b) and the Polyporus umbellatus (order Polyporales) has a genetic diversity of 0.544 (Liu et al 2015). The high genetic diversity observed in G. boninense, along with the small number of duplicated genotypes found in our sample, indicate that sexual reproduction plays a key role in spreading the disease, as proposed by Miller (1995), Pilotti et al (2003), and Rees et al (2012), Despite the fact that some palms could be infected by multiple genotypes (Miller 1995;Ariffin et al 1996;Pilotti et al 2003;Rees et al 2012) and that we only collected one fruiting body at random from each palm, the small number of duplicate genotypes we found (less than 2 %) suggests that disease spreading by root-toroot infection is anecdotal. Duplicate genotypes were always collected on neighbouring trees about 9 m apart, while oil palm roots are known to extend up to 30 m in general (Jacquemard 2011).…”

“…Broad-scale dispersal is common in many basidiomycetes (Hallenberg & Kuffer 2001). Pilotti et al (2003), and more recently Rees et al (2012), have shown a gene flow at the estate level, but our PCA results hint at a broader scale of gene flow embracing both Sumatra and peninsular Malaysia. Results from Bayesian assignment tests in STRUC-TURE and AMOVA support the hypothesis that isolates from both regions originate from a single population, which indicates continual gene flows between those two regions in spite of the Malacca strait.…”

“…The spreading pattern of G. boninense appears to be very different from A. mellea, another model of perennial crop root rot basidiomycete, which has a wide host spectrum and mainly spreads by vegetative multiplication (Prospero et al 2008;Baumgartner et al 2010;Travadon et al 2012 b). As previously shown by Pilotti et al (2003), G. boninense's dispersal mechanism seems more similar to that of Heterobasidion annosum, a Polyporales-like white rot fungus which is responsible for serious losses in conifer forests in the northern temperate regions (Woodward et al 1998) and has a predominantly sexual reproduction pattern.…”

“…Since then, much effort has been devoted to studying the oil palm's defense mechanisms against G. boninense infection and to developing BSR resistant or tolerant varieties (Idris et al 2004;Durand-Gasselin et al 2005;Tee et al 2013;Ho & Tan 2014). While some studies have attempted to improve understanding of the biology and diversity of G. boninense by describing its morphology in detail (How & Nawawi 1985), characterizing its modes of sexual reproduction (Pilotti et al 2002) and quantifying its genetic diversity with molecular markers (Miller et al 1999) or mating type systems studies (Pilotti et al 2003), its genetic diversity and population structure across oil palm plantations in Southeast Asia are still poorly understood.…”

“…The obtained data were used to address the following questions: (i) Does sexual reproduction through basidiospores play the same role for G. boninense in oil palm plantations from peninsular Malaysia and Sumatra as it does in Papua New Guinea (Pilotti et al 2003) ? (ii) Does the Malacca strait cause a significant barrier to G. boninense gene flow?…”