Quorum sensing-mediated inter-specific conidial anastomosis tube fusion between Colletotrichum gloeosporioides and C. siamense

Abstract

Many plant pathogenic filamentous fungi undergo fusion of conidia through conidial anastomosis tubes (CATs), which is believed to facilitate horizontal gene transfer between species. We discovered a remarkable inter-specific CAT fusion between two important plant fungal pathogens Colletotrichum gloeosporioides and C. siamense. In an invitro assay, under no selection pressure, the inter-specific CAT fusion was preferred with higher frequency (25% ± 5%) than intra-specific CAT fusion (11% ± 3.6%). Different stages of CAT fusion viz. CAT induction, homing, and fusion were observed during this inter-specific CAT fusion. The CAT fusion was found to be higher in absence of nutrients and under physiological stresses. This CAT fusion involved a quorum sensing phenomenon, wherein the CAT induction was dependent on conidial density and the putative quorum sensing molecule was extractable in chloroform. Movement of nuclei, mitochondria, and lipid droplets were observed during the CAT fusion. Post CAT fusion, the resulting conidia gave rise to putative heterokaryotic progenies with variable colony characteristics as compared to their parental strains. Few heterokaryons showed variable AFLP banding pattern compared to their parental strains, thereby suggesting a possible genetic exchange between the two species through CAT fusion. The heterokaryotic progenies exhibited varied fitness under different stress conditions. Our study illustrated a possible role of inter-specific CAT fusion in generation of genetic and phenotypic diversity in these fungal pathogens.

Differential Physiological Prerequisites and Gene Expression Profiles of Conidial Anastomosis Tube and Germ Tube Formation in Colletotrichum gloeosporioides

“…The heterokaryotic progenies have been generated through CAT fusion with distinct phenotypes compared to parental strains in F. oxysporum and C. lindemuthianum [9,13]. Inter-specific CAT fusion between C. lindemuthianum and C. gossypii resulted in the formation of some hybrids, which exhibited phenotypic variations; one hybrid was highly pathogenic [14] Furthermore, we have demonstrated that an inter-specific CAT fusion between C. gloeosporioides and C. siamense generated genetic and phenotypic diversity in these fungal pathogens [11]. Pathogen populations with higher genetic variations tend to show increased mean fitness, resilience, expansion of host range, and fungicide resistance.…”

“…To study the effect of conidial age on GT formation and CAT fusion and their mutual exclusiveness in C. gloeosporioides, the GT and CAT induction assays were carried out by following a protocol developed in our lab [11]. The C. gloeosporioides (CBS 953.97) culture was inoculated on bean pod agar medium (autoclaved French bean pods submerged in 2% water agar) and was incubated in the dark at 25 • C to induce conidiation.…”

“…In contrast, for CAT induction, different concentrations of 17-day-old conidia were incubated in dH 2 O for 72 h in the dark. The concentrations of conidia tested were 4 × 10 2 , 4 × 10 3 , 4 × 10 4 , 4 × 10 5 , and 4 × 10 6 per mL [11].…”

“…The CAT fusion versus GT formation were independent of conidial age in N. crassa and F. oxysporum [4,5,9]. Interestingly, older conidia were optimal for CAT fusion while younger conidia were suitable for GT formation in C. lindemuthianum, C. gloeosporioides, and C. siamense [10,11]. The CAT fusion might play a vital role in the life cycle of fungi by improving the chances of colony establishment during nutrient starvation [7].…”

“…gloeosporioides starts its usual life cycle from conidial germination to the formation of melanized infection structures, appressoria [15]. The formation of GTs and CATs in C. gloeosporioides were reported in both in vitro and in vivo conditions [11,16]. Despite having legitimate morphological variations between GT and CAT, their whole transcriptome comparison has never been reported.…”