TERRITORIAL URBANA
C) Resultados y Discusión
Many marine organisms, including gastropods, show either direct development in which eggs hatch into crawl-away juveniles or they have planktonic larval development (Weersing and Toonen, 2009). As the adult stage is generally not highly mobile, dispersal is mainly achieved during the pelagic larval phase. Therefore, species with planktonic larval stages are expected to show less genetic structure compared to species without larval stages (Weersing and Toonen, 2009). Additionally, the type of larva, which determines the pelagic larval duration (PLD), may also define the extent of the dispersal ability of the larva and level of mixing between distant populations (Silva and Russo, 2000). There are two distinct larval types in invertebrates. Planktotrophic larvae that hatch from small eggs develop slowly because they must feed to complete the larval stage. Lecithotrophic larvae hatching from large eggs can complete the stage faster (McEdward, 1997). Despite some conflicting findings (Weersing and Toonen, 2009), there is a positive relationship between the PLD and the strength of the genetic connectivity between populations (Selkoe and Toonen, 2011). When the PLD is short, other biological or environmental factors have a stronger effect on the genetic structure of the organism compared to species with long PLDs (Ross et al., 2009). It is worth noting that the effect of the PLD is attributed more to passive movements due to tidal and wind movements than the active movement of the larvae (Gardner et al., 2010). P. maculata lays multiple small eggs (100 µm) encapsulated in a gelatinous
cylindrical mass (Figure 1.1B). The eggs hatch into larvae within ten days (Gibson, 2003; Wood et al., 2012b). The planktotrophic larvae feed on plankton for approximately 3 weeks before settling on bio-filmed surfaces as juveniles (Gibson, 2003; Wood et al., 2012b). This relatively long PLD can increase genetic homogeneity among P. maculata populations. 1.3.6 Patterns of sexual reproduction
Several patterns of sexual reproduction are observed in invertebrates, and the pattern of sexual reproduction determines the structure of the populations to some extent. These
21 patterns are biparental reproduction, parthenogenesis and hermaphroditism.
Hermaphroditism is the focus in this study because it is the reproduction pattern of P. maculata (Willan, 1983). Hermaphroditic organisms have both male and female
reproductive organs during their life span. Hermaphroditic organisms can reproduce by autogamy or self-fertilisation, which will lead to structured populations (Charlesworth and Wright, 2001). Conversely, outcrossing simultaneous hermaphroditic invertebrates have a large effective population size and show high levels of genetic variation (Silva and Russo, 2000). P. maculata is a simultaneous hermaphrodite in which reciprocal copulation is often observed (Willan, 1983). In this case, high genetic variation can be expected for this
species.
To summarise, this chapter has identified several biogeographic barriers that affect genetic connectivity in populations of NZ coastal organisms. The effect of these barriers can vary from species to species based on the life history and evolutionary responses of the
respective species to various geological changes and other environmental factors (Ross et al., 2009). Therefore, the potential divergent effects of these barriers should be taken into consideration in a species-specific manner for studies that address the issue of population structure for any NZ marine species, including P. maculata.
1.4
THESIS SUMMARY
The origins and evolution of toxic P. maculata individuals have not been clarified. Identification of distinct geographical regions that contain either toxic or non-toxic individuals raises certain questions. For example, has a toxic population recently invaded NZ’s coasts? Are individuals from toxic and non-toxic regions genetically isolated, or are they even cryptic species? What is the degree of genetic connectivity between distinct populations, if there is any? Population genetic tools can address these questions when combined with the current knowledge of phylogeographic barriers that are effective in several NZ marine species. It will also help to understand the underlying evolutionary forces that shape the population structure of P. maculata. My PhD project had three overall aims: 1) to understand the spatial distribution of genetic diversity across P. maculata
22 populations from various regions in NZ utilising microsatellite and mitochondrial data; 2) to understand the genetic connectivity of different P. maculata populations, and determine the biogeographical factors shaping its population structure; 3) to determine whether there is a correlation between variability in TTX concentrations and genetic structure, or not. With these goals in mind, I developed microsatellite markers, which was a significant first step with this intractable organism. The development and validation of these markers is described in Chapter 2. Chapter 3 deals with thematerials and methods used in this study. I used microsatellite markers that I developed to investigate the diversity and genetic structure of the P. maculata populations around NZ, the results of which are presented in Chapter 4. Chapter 5 presents the analysis of the population structure of P. maculata
again, but this time utilising sequences from mitochondrial DNA genes, including COI and Cytb, and discusses the results found from both marker types (microsatellite and
mitochondrial). The species-specific markers employed for this step were also developed. The final Chapter (6) contains concluding remarks and future perspectives. The toxicity assay was performed by our collaborators (Susanna Wood and Paul McNabb) at Cawthron Institute (Nelson, New Zealand) for not all, but only a proportion of the samples. However, the general trend of toxicity levels at each locality was known based on previous studies ( (McNabb et al., 2014; Salvitti et al., 2015a; Wood et al., 2012b) or personal
communications (Susanna Wood). Samples from the northern North Island (Ti Point, Coromandel, Auckland and Tauranga) were considered as toxic whereas relatively southern regions (Wellington and Nelson) were considered as slightly toxic or non-toxic. One
interpretation of these data suggests that there is an association between toxicity and
population structure, however this is a correlative association and does not constitute proof. Nonetheless, the correlation raises important hypothesis that can be tested directed in future studies.
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