La gestión por competencias: como herramienta directiva

The observed pattern of population structure resolved along the Thai-Malay Peninsula for both genetic and morphometric datasets was largely incongruent with the pattern expected for two independent populations bisected by a historical biogeographic barrier. In other words, there were no strong signals from the present study to support the existence of divergent melon fly populations on either side of the Isthmus of Kra barrier. This is consistent with studies of the Oriental fruit fly (B. dorsalis), for which populations are unstructured along the length of the Isthmus of Kra (Krosch et al., 2013; Akrawong et al., 2014).

Understanding of the biogeographic history of the Kra Isthmus is becoming increasingly complex. Studies of animal taxa across this region show a range of patterns and possible transition zones. These alternative biogeographic transition zones are possibly driven by historically unstable climate effects (Pauwels et al., 2003; Hughes et al., 2011) or multiple past marine transgressions into the Thai- Malay Peninsula (Woodruff, 2003; de Bruyn et al., 2005). Taken together, the data presented here add one more piece to the larger biogeographic puzzle by reporting a population-level study based on markers that also provide data relevant to biogeographic regions beyond the Thai-Malay peninsula.

It has been suggested that there was a trans-peninsular seaway that acted as a barrier and permitted divergence of the Indochinese and Sundaic biotas at the Isthmus of Kra (Woodruff, 2010). The transitions in some groups of animals lie north

Chapter Three 150 of Kra in the northern peninsula, but these are not particularly well documented. A biogeographic barrier at the Isthmus of Kra is indicated by the zoogeography of several groups of animals, especially amphibians, fish and crustaceans, which cannot move far away from their habitat (Inger, 1966; Benzie, 1999; Inger, 1999; de Bruyn et al., 2005; Crandall et al., 2007). Studies of total regional faunas, e.g., birds (Hughes et al., 2003; Round et al., 2003) and honey bees (Rueppell et al., 2011), also support the concept of a single biogeographic barrier at the postulated Kra Seaway. However, other studies show no biogeographic barrier effect of the Kra Seaway (e.g., Pramual et al., 2005), nor do they identify alternate zones of faunal and/or population transition further north or further south (Wikramanayake et al., 2002; de Bruyn et al., 2005; Woodruff & Turner, 2009; Patou et al., 2010; Hughes et al., 2011). These latter faunistic studies, which downplay the significance of the Isthmus of Kra, correspond to geological data which suggests that the peninsula has never been dissected by a complete seaway (De Bruyn et al., 2005). If not Kra, then are there other barriers? A cluster of genetically divergent and reciprocally related populations from Yala, Narathiwat and Selangor fits with the idea of a biogeographic barrier at the Kanger-Pattani line. Most historical explanations for the current distributions of Southeast-Asian biota invoke the existence of ancient seaways during periods of high sea levels, with these seaways forming barriers to dispersal by terrestrial organisms (Woodruff, 2003). This is not what is thought to have happened at the Kanger-Pattani line, with the marked turnover of species at the Kanger-Pattani line postulated to result from a corresponding change in environmental conditions (Whitmore 1984; Ashton, 1997; Lohman et al., 2011).

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Figure 3.36 Maps showing location of the Kanger-Pattani biogeographic barrier

(modified from Lohman et al., 2011 and Baltzer et al., 2008).

The Kanger-Pattani transition is demonstrated by a major floristic and climatic transition from seasonal dry forest to aseasonal evergreen forest (Baltzer et al., 2008; Woodruff & Turner, 2009; Lohman et al., 2011). Approximately 575 plant genera reach their distributional limits at this line (Van Steenis, 1950; Lohman et al., 2011). Moreover, the distributions of many mammals, including bats, support a transition zone at the Kanger-Pattani line (Hughes et al., 2011). The close correspondence between species distributional limits and a rainfall seasonality transition has been proposed as a primary mechanism maintaining species distributional limits at the Kanger-Pattani line (Whitmore, 1984; Ashton, 1995; Richards, 1996; Lohman et al., 2011). Thus, both historical and environmental mechanisms could be invoked to explain the distributional pattern involving the Yala, Narathiwat and Selangor populations of Z. cucurbitae.

Local adaptation due to environmental variation

The morphology of flies from Nan and Chiangmai (northernmost populations) was different from others. The northern region is composed of several mountain ranges of high altitude, and this topography may separate populations of melon flies. As previously noted, there are some populations of guava fruitfly,

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Thailand (Kunprom et al., 2013). Kunprom et al. (2013) related the distinctive population of B. correcta from Phetchaboon province to environmental variables of the Phetchaboon range. Notwithstanding the subtle morphological differences of the northern melon flies and the genetic distinctness of melon fly from the northeast (noted above), Z. cucurbitae from the North and Northeast do not differ markedly from those from elsewhere in Thailand. The determining factor for the melon fly may be the fact that the cucurbit host-plants for Z. cucurbitae are very common in Thailand and effectively present a continuous habitat for the fly, promoting gene flow among populations.

Environmental factors could account for at least some of the affinities between the flies at Yala, Narathiwat and Selangor populations, which were significantly different from all other local populations. It is also worth noting that all of these three site were geographically adjacent to large tracts of natural evergreen forest. Thus, it could be that the characteristics which make flies from these sites distinctive from other populations, and similar to each other, were derived from

flies developing in wild host plants. Many host-plant species reported for

Z. cucurbitae (Allwood et al., 1999) are native plant species commonly found in the

natural forest in Thailand.

Conclusion and link to next chapter

Two factors are most likely to account for the genetic homogeneity among populations of Z. cucurbitae in Thailand. First, Z. cucurbitae utilizes a wide range of host plants in Southeast-Asia, including 42 plant species belonging to 20 families (Allwood et al., 1999). Many of these host plants (e.g., cucurbits and beans) are commonly grown in Thailand, and so melon fly populations are likely to be geographically continuous. Second, human-mediated dispersal, such as local transportation of fruit and trade, may also facilitate movement of flies, as has been reported in other fruit fly species (Malacrida et al., 2007; Shi et al., 2012). This movement of flies with fruit would promote genetic exchange (i.e., gene flow) between populations. This gene flow would counter the effect of genetic drift or selection by lowering the level of genetic differentiation. The overall genetic structuring in Thailand was low except for the Northeastern population, which is

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physically isolated and ecoclimatically distinct from other Thai regions. The haplotype shared between the mainland and islands most likely arose from long distance migration, which could be a result of either the historical population expansion or human-mediate gene flow.

These data do not suggest the presence of a cryptic species complex within

Z. cucurbitae in Thailand. However, to conclusively test this, the next chapter

examines host plant effects on morphological and genetic variance in Z. cucurbitae. Focusing on a single region in Thailand, it examines the variance of flies reared from known hosts, comparing particularly flies reared from cucurbits versus those reared from non-cucurbit hosts. It thus examines for the likelihood of host races occurring in this species.

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CHAPTER 4

Cryptic host races in Zeugodacus cucurbitae: little evidence of

host-related structure in Thailand

Chapter Four 156 4.1 INTRODUCTION 4.1.1 Environment effects on insect variation

Organismal variability derives from the combined effects of the genotype and environmental influences and individual experiences during development (Beebee & Rowe, 2008). The environment an organism experiences can affect which genes are expressed, the extent to which they are expressed, or how genes interact to result in a particular phenotype (Nijhout, 1999; Beebee & Rowe, 2008). The most visible manifestation of this, especially for the insects, is represented by variation in morphology (Roff & Fairbairn, 1991; Zera & Denno, 1997; French et al., 1998). Environmental variables, such as temperature, nutrition, photophase and humidity, may also influence growth and development and so, ultimately, affect population fluctuations as well as phenotype (Moczek, 1998; Yin et al., 2007; Chown & Gaston, 2009). Temperature may play a particularly strong role in influencing insect body size (Partridge & French, 1996) with, for example, increased development time at lower temperatures typically resulting in larger adults (Stern & Emlen, 1999). In addition to this, nutrition during development is widely considered the key factor influencing adult size and allometry (Moczek, 1998; Awmack, & Leather, 2002; Shingleton et al., 2007). This effect may be intergenerational, as illustrated by the gypsy moth, Lymantria dispar (Linnaeus), in which the nutritional environment can significantly influence both the growth and reproductive potential of the subsequent generation (Lindroth et al., 1997).

Frugivorous tephritids oviposit into fruit, in which eggs hatch and larvae feed. Indeed, it is a specific characteristic of fruit fly larvae that they are internal feeders of fruit and that they develop in the host fruit selected for oviposition by the female parent (Averill & Prokopy, 1987; Novotny et al., 2005); as a result, other environmental effects on the larvae are limited and may play a relatively reduced role in development. Accordingly, the most significant environmental effect on tephritids comes from the host plant (Bush, 1974, 1992; Feder et al., 1988; Awmack, & Leather, 2002). Since the host plant is such a determining factor in the development of tephritids, these flies are prime candidates to diversify into genetic

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lineages (be they host races and/or cryptic species) which are associated with different host fruit.