Pests and diseases

One of the most important contributions that molecular ecology can make towards the control of pests and diseases is tracking the source of an outbreak and the subsequent dispersal patterns. The spread of a pathogen tends to occur either as the result of a rare isolated event involving long-distance dispersal, or following a pattern of gradual expansion in which the range of the pathogen increases in an ongoing and more predictable manner (Brown and Hovmller, 2002). Differentiating between the two scenarios may help us to control the spread of pathogens and can sometimes be accomplished through an analysis of population genetic structure. This is because, as we saw in Chapter 5 during our discussion of postglacial dispersal routes, founder effects should be most pronounced following rare long-distance dispersal events, and therefore populations that have expanded in this manner are expected to show relatively low levels of genetic diversity and relatively high levels of population differentiation.

Researchers applied this theory when investigating the ascomycete fungus Mycosphaerella fijiensis, a pathogen that causes the very serious black leaf streak disease in bananas. Believed to have originated in southeast Asia, this fungus is spreading around the world. Several PCR-RFLP markers were used to compare levels of diversity and overall genetic similarity within and among M. fijiensis populations from Latin America, the Caribbean and Africa. Founder effects were evident at the scales of continents and countries, suggesting that the invasion of new countries resulted from infrequent long-distance dispersal. This is compatible with either limited long-distance airborne dispersal of ascospores or the movement by humans of infected plant material over long distances. This finding suggests that improved quarantine measures may help to limit the spread of this costly pathogen (Rivas et al., 2004).

Even if a pest is already widespread, it may still be important to understand dispersal patterns because of the potential for the later spread of novel strains that are resistant to pesticides. Since the 1940s pesticide resistance has evolved repeatedly, with over 500 species of insects and mites no longer susceptible to a range of chemicals (Georghiou, 1990). Pesticide resistance was an important reason for studying dispersal in the potato cyst nematode Globodera pallida, which parasitizes potato roots and can greatly reduce potato harvests. Larvae actively disperse through the soil over short distances, and dormant cysts containing several hundred larvae can passively disperse through water run-off or humanmediated transport of soil. Gene flow in potato cyst nematodes was for many years believed to be negligible, although this has been contradicted by a recent study in which researchers used PCR-RFLP markers to quantify gene flow at three different scales in Peru: within fields, between fields (separated by 3--35km), and between regions (separated by 326-832 km). Although the regions were genetically distinct from one another, gene flow was extensive both within fields and between fields within the same region (Figure 8.3). This is most likely attributable to the passive cp a E o a

Figure 8.3 Pairwise FST values between populations of the potato cyst nematode at local and regional scales. Gene flow is extensive both within fields and between fields within the same region, although regions remain genetically distinct from one another. Adapted from Picard et al. (2004)

Figure 8.3 Pairwise FST values between populations of the potato cyst nematode at local and regional scales. Gene flow is extensive both within fields and between fields within the same region, although regions remain genetically distinct from one another. Adapted from Picard et al. (2004)

dispersal of cysts, which can remain dormant for up to 10 years. These findings suggest that potato farmers could limit the spread of G. pallida by disinfecting seed tubers used for planting, by regularly cleaning farm machinery, and by limiting the spread of cysts through irrigation water (Picard et al., 2004).

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