One interpretation of the data suggests that the population genetic structure of P. maculata correlates with the regional variations in TTX concentration: the northern North Island populations contain highly toxic individuals and are significantly different from the WL and NL populations, which harbour either slightly toxic or non-toxic populations, respectively. In addition, weak differentiation was observed between the
WL and NL populations. Population structure and migration analysis based on
allozymes, mtDNA (Kuchta and Tan, 2005) and microsatellites (Ridenhour et al., 2007) for the TTX-bearing Taricha granulosa newt from various localities in western North America shows little differentiation and high gene flow between populations that exhibit large phenotypic variations. Similarly, phylogenetic analysis based on mtDNA markers for the TTX-containing red-spotted newt Notophthalmusviridescens shows that highly toxic and non-toxic populations (Yotsu-Yamashita et al., 2012) are genetically identical, and the authors suggested an exogenous source for TTX. P. maculata’s
population structure correlating with TTX concentrations does not resemble these examples.
The correlation with toxicity and population genetic structure in P. maculata could be interpreted as indicative of a genetic mechanism underlying the phenotypic variation. However, the capacity of P. maculata individuals from non-toxic regions to accumulate high concentrations of TTX (Khor et al., 2014; Wood et al., 2012a) and rapid depuration of TTX when the individuals are fed on non-toxic diet (Wood et al., 2012a) suggests an external TTX source. This implies that geographical variations in toxicity are more likely associated with environmental differences at distinct geographical locations.One possible explanation is the geographical variations in the distribution and/or abundance of P. maculata’s toxic food sources. For example, a flatworm, Stylochoplana species, which is also a prey for P. maculata in the Tauranga region, was also found to contain high concentrations of TTX (Salvitti et al., 2015a). The toxicity level between
individuals of this species also shows significant differentiation as found in P. maculata. Seasonal variations in TTX concentrations in Stylochoplana sp accord with those of P. maculata, and mass calculations show that P. maculata might accumulate the observed TTX concentrations by preying on Stylochoplana sp. However, this Stylochoplana sp. has not been found in geographical areas where other toxic P. maculata individuals are observed, implying that Stylochoplana sp. is not the only TTX source for P. maculata
(Salvitti et al., 2015a). Additionally, no other common source that has enough TTX concentration has been identified in other regions associated with toxic P. maculata
individuals. However, the possibility that other toxic dietary resources have not been discovered still exists (Khor et al., 2014).
Another possibility – if the ultimate source of TTX is biosynthesis by one or more commensal microorganisms – is that certain environmental factors may affect the differential distribution of the commensal or symbiont (Wood et al., 2012b). Ultimately, a bacterial origin of TTX may explain the presence of TTX in other organisms such as
Stylochoplana sp. The population structure of P. maculata revealed by microsatellites seemingly correlates with geographical barriers that result in the north-south disjunction although my limited sampling did not allow me to study the possible effects of the seascape features around NZ on the structure of P. maculata populations. If this is the case, the geographical barriers may also affect the distribution of such microorganisms, which would indirectly result in a correlation with toxicity and population genetic structure in P. maculata.
Episodes of regionally confined TTX poisoning such as dog poisoning events that occurred on Auckland beaches in late 2009 (McNabb et al., 2010) are not unique to NZ. In 2007, TTX-poisoning events occurred due to consumption of Charonia lampas, which is a gastropod species native to Mediterranean and Atlantic waters (Rodriguez et al., 2008). Silva et al. (2012) investigated TTX levels in several gastropod, bivalve and starfish species sampled from the Portuguese coasts between July 2009 and November 2010. They detected low concentrations of TTX or its derivatives in two native edible gastropod species, Gibbula umbilicalis and Monodontalineata. Based on the
identification of putatively TTX-producing Vibrio species in puffer fish only at warm temperatures (20–29˚C) (Sugita et al., 1989), Silva et al. (2012) argued that increased sea temperatures in the Mediterranean Sea may facilitate the growth of TTX-producing bacteria, and result in TTX-production in native gastropod species (Silva et al., 2012). Differences in sea surface temperatures between the North and South Islands might have also led to the clear toxic and non-toxic population cut-off between the islands (Wood et al., 2012b).
Further studies on additional populations of P. maculata will help to reveal the exact location of the north-south population differentiation, the existence of other
differentiated populations (if any), and various additional factors shaping population structure. Even though I found a correlation between population differentiation and TTX concentrations, my data are unable to say anything conclusive about the origin of TTX in P. maculata. It is important to note that the limited sampling might have also
given an impression of the correlation although it does not exist. Studies that aimed to identify TTX-producing microorganisms have not found any evidence supporting a culturable bacterial origin of TTX in P. maculata (Chau et al., 2013; Salvitti et al., 2015b). On the other hand, there is strong evidence for a dietary origin of TTX for at least some regions in NZ (Khor et al., 2014; Salvitti et al., 2015c). The ultimate source of TTX in P. maculata as well as other TTX-containing organisms is most probably biosynthesis by one or more commensal microorganisms that are not culturable or that cannot produce TTX in-vitro (Chau et al., 2013; Salvitti et al., 2015c). As a further step, the metagenomic approach may reveal P. maculata’s associated microflora, and
accordingly unculturable microorganism taxa. Additionally, the metagenomic approach may help to identify putative gene clusters that could be involved in biosynthesis of TTX (Gerwick and Moore, 2012).