General discussion

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CHAPTER 6

General discussion and conclusions

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sides of Ganges river. Oecophylla species may disperse via nuptial flight of queens and/or rafting method which is very effective between island to island dispersal (Peng et al. 1998).

The result of distribution of mtDNA types shows that the western parts of Bangladesh is predominantly occupied by the Indian type, whereas the eastern part by the SE Asian type, and that the central parts is the mixture of both types. The study does not show the clear boundary, but suggests the central part as a transition zone. The haplotype network analysis implys the diversity within each of the types: the Indian type includes 14 haplobtypes and the SE Asian type includes 11 haplotypes.

Effects of temperature and rainfall influence the distribution of O. smaragdina as the combination of high temperature and high rainfall is necessary to continue the weaver ant growth rate and distribution, and to a limited extent, higher temperatures can compensate for lower than optimal rainfall levels, and vice versa (Lokkers, 1986). During the glaciation period the temperature had fall down nearly 10 ^oC in Vostok (Lowe and Walker, 1997) might have significant influence of the distribution of several species like Oecophylla, which is markedly affected by low temperature for their colony development. The haplotype network distribution pattern (in chapter 4) also showed that the bottleneck effects might be responsible for such distribution. Due to several glaciation during Pleistocene could be a key factor of such effects.

Weterrer (2017) showed a comprehensive distribution pattern of O. smaragdina in the arid, tropical and subtropical climatic region that also shed lighted the effects of temperature on the distribution.

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A hypothesis forming the distribution pattern

The occurrence of two types (Indian type and SE Asian type) in Bangladesh will be made by the following dispersal scenario: The disjunction of populations would occur several times as in shown of divergent time in 7 major groups of mtDNA, and the isolated populations may develop their unique haplotypes in India, in SE Asia, or others during the history of O.

smaragdina which was estimated to diverged from longinoda about 13 - 11 Ma (Azuma et al.

2006; Blamer et al. 2015).

Before the Last Glacial Maximum (LGM: 25,000 - 15,000 BP) the haplotypes observed in Bangladesh would be already established. During the LGM, the Bengal Delta supposed to be vacant for Oecophylla smaragdina distribution because the northern limit of the species would be located down to the south. After the LGM, the populations expanded to north, and because of the geographical shape the east and west populations met in the Bengal Delta. When the two types of population groups met again in the Bengal Delta, each of the types already contained several haplotypes of mtDNA.

This vacant-reunion hypothesis requires a temperature drop in LGM, and diversification of populations before LGM. The present distribution of Oecophylla smaragdina, in particular, the northern limit of distribution is almost agree with the isothermal line of 10 ^oC in the average temperature of January. Bangladesh has a distinct monsoonal season, with an average temperature in January of 18 ^oC in Dhaka. Thus if the mean temperature of the coldest month (MTCO) drops 8 ^oC or more, O. smaragdina cannot distribute in Bengal Delta. Braconnot et al. (2012) estimated the mean temperature of the coldest month (MTCO) in Asia is around 10

oC below. Although northern limit of distribution of O. smaragdina in LGM is not clear, the

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Bengal Delta is supposed to be out of the limit. After LGM, as the climate changed to warmer, the populations would extended to northward and come to Bengal Delta simultaneously. This is based on the result from the present study which revealed that there are14 haplotypes in Indian type and 11 haplotypes in SE Asian type. It is almost certain that the Indian type from west or southwest, and SE Asian type from east or southeast met in Bengal Delta. The populations extended to Bengal Delta were not uniform, because each of mtDNA types have a history of diversification in somewhere south in India and SE Asia. The diversification time was estimated in the Pliocene to Pleistocene.

The recent phylogeographic studies on insects, plants, amphibians or fishes shown the significance of Pleistocene climate changes and biogeographic barriers like mountains, rivers, seas and desserts alter the diversification, radiation and isolation of new genetic lineages within many species (Riddle et al. 2000). This climatic oscillation played the major role of forming such distribution of species by changing the genetic structure and diversity (Avise, 1994).

Regarding the regions of East Asia, the glacial influence were not so extensive due to influence of monsoons formed by the Pacific Ocean and the presence of biotic zone at higher northern latitude. However, the fossil and biogeographic evidence showed the dramatic effects of climate changes thorough Asia and its impact on the distribution of several animal species is under consideration. The case of the distribution pattern of O. smaragdina in Bangladesh can be correlated with the distribution pattern of Asian elephants. Vidya et al. (2009), explained that in the case of the elephant haplotypes in Myanmar, rather suggested tha these haplotypes did not arise within Myanmar, but instead resulted from a northward range expansion of beta clade haplotypes during warm period from both Sri Lanka and the Sunda region followed by subsequent admixture in this region. The geological history of Bangladesh revealed that the formation has begun 350 million years ago when the Pangean supercontinent broke apart

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(Mannan, 2002). The Bengal Basin, which is the major basin of Bengal formation, began 127 million years ago when the Indian Plate rifted away from Antarticaat 18 cm per year for 20 million years. So, after the age of glaciation, as a new vacant area with newly growing vegetation favored by the tropical climate facilitate the chance of occurrences of both the types.