Spot blotch has emerged as one of the most important pathogen particularly with regards to wheat and barley production throughout the world inclduingn south east Asia [Gupta et al. 2018]. Host resistance is economic and vaibale approche for managing the pathogen. However, these measures have not yielded satisfactory results thus far. Spot blotch variation is extremely useful for identifying resistant germplasm, deploying cultivars with different resistance genes, and analyzing the emergance of new pathotypes. A genetically diverse pathogen population screening is required for an effective resistance breeding programme. As a result, understaning the natural pathogene genetic variation is critical for developing effective management strageties. Moreover, genetic variability among naturally occurring populations is an outcome of evolution and demographic processes and provides an important tool to determine the evolutional potential of a species [Zhang et al 2017]. Here, we examined the intra-specific genetic variability of B. sorokiniana (using ITS, GAPDH and TEF1α region of B. sorokiniana) to acknowledge the evolution and phylogeography of the species.
According to phylogenetic study, B. sorokiniana was a single species with no distinct groups, since all of the sequences taken from different countries were clustered together in a single group and outgroups were put in separate group. Haplotype diversity is influenced by mutation, marker discovery, recombination, and demography [Stumpf 2004]. In present study, 40 haplotypes have been observed in a group of 254 isolates based on mutli-locus gene sequence with predominant haplotype H_1 comprising of 127 individuals (50% of total isolates) with distinct geographies and host specificities. These results were indicative of high gene flow occurring among the geographically distinct populations of the pathogen. The haplotype network has a dispersed topology due to its high singleton ratio. This kind of phylogenetic inference is indicative of a recent population expansion that evolved from a small number of founders following a genetic bottleneck [Zhang et al. 2017]. The results of median-joining network analysis of B. sorokiniana isolates revealed that few haplotype were population specific and unique while others were evolutionarily distinctive (H_1) and exhibited significant levels of gene flow among populations. The Asian haplotypes H_2, H_4, H_6, H_7, H_12, H_15, H_16, H_17, H_18, H_19, H_20, H_21, H_22, H_23, H_25, H_26 and H_39 appeared to have evolved from the predominant haplotype H_1 (Fig. 1). The haplotypes H_1, H_3, H_4, H_27, H_28, H_29, H_30, H_31, and H_40 were from barley isolates of India, Morocco, Syria, Turkey, Japan, China and Syria origin respectively (Table 1). We found that Indian isolates were diversed and spread in the network in 16 haplotypes i.c. H_1, H_2, H_5, H_9, H_10, H_11, H_12, H_13, H_14, H_29, H_30, H_33, H_34, H_35, H_36, and H_37. A star-like structure of the median joining network discovered in this study also offers evidence to support the presumption of divergence from neutrality for constant or uniform population size (Fu's Fs and Tajima's D) owing to spot blotch fungal population expansion. Introduction, domestication, and continuing cultivation of B. sorokiniana host plants may therefore be a feasible explanation for population increase in various wheat growing zones. This form of fungal population mobility has been seen in a range of fungal diseases [Dietzel et al. 2019; Zaffarano et al. 2008; Katoch et al. 2016].
The indices were non-significantly positive in Indian group, showing divergent demographic histories among analysed regions. Tajimas’D is negative for most of the groups but statistically positive and Fu’s Fs were negative for only Syria but statistically significant, indicating that the B. sorokiniana populations did not match a simple model of neutrality and rejected the null hypothesis of constant population size. These findings postulated that the divergence from neutrality for constant population size (Tajima’s D and Fu’s Fs) was caused by the recent population diversification. The introduction of B. sorokiniana might be the cause of the species recent population expansion, as Bipolaris now exhibits pathogenic specialization on a variety of wild and cultivated host plants. This type of range expansion has also been reported in many fungal and insect species [Katoch et al. 2016; Prabhakar et al. 2012; Wang et al. 2016]. The dating analysis shows that the species B. sorokiniana originated from Middle Miocene era (~ 19 million years ago). The Miocene era had the longest and most humid environments in the geological history. According to Palaeogeographic analysis, the western part of central Asia transitioned from a humid climate to a semi-arid climate [Lauer et al 2017; Zhang and Zhang 2012]. This climatic change is thought to have accelerated speciation due to habitat alternations caused by progressive acidification and the quaternary glacial-interglacial cycles. As a result, the climatic change results in contraction and speciation. Divergence times estimated for the primary genetic breaks were placed within the Plio/Pleistocene periods (~ 2 mya) (Fig. 4), a period in which the average temperature of the earth began to decrease rapidly and for which extensive records document changes in forest distribution associated with climatic cycles. The first break among the groups occurred in Asian lineages of B. Sorokiniana (the oldest ones). From this study, we also came to know that three sequences from Asia (KM093765, KJ939504 and KJ939501) clustered with P. herbarum outgroup in phylogenetic tree which indicate that these sequence might have evolved from P. Herbarum since 15.6 million years ago.