Characterization And Evolution Of Reverse Transcriptase Sequences Of Retrotransposon Ty1-copia From Eight Species Of Fig Wasps

Fig wasps (Insecta:Hymenoptera:Chalcidoidea) and their host figs (Moraceae:Ficus) are classical materials for the study of symbiotic relationship between plant and insect. A syconium always shelters more than two species of fig wasps. Generally, there is only one species of pollinator with an obligate mutualistic relationship with its host tree, while the others are non-pollinating fig wasps. Due to their differentiated ecological niches inside the syconium, the different species of fig wasps in the system may be under divergent selective pressures, and the pollinators are prone to be under stronger pressures considering their closer relationship with the figs.Transposable elements (TEs), also called transposons, are genetic entities which can move along chromosome, and they are ubiquitous among various eukaryote genomes. Early in the1960s, most researchers considered TEs as "selfish elements" or "junk DNA" owing to the higher duplicate frequencies (always leading to a reduced fitness of hosts) of these elements compared to the host genes. However, during the last two decades scientists have gradually accepted the evolutionary innovation roles of TEs. First, there is a clear correlation between the transposition activities of TEs and the sizes of the host genome. Second, the recombination events caused by transposition will always play an important role in the reshaping of host genome structures. Third, the recombination and transposition will also lead to diversities of mutations in the host chromosomes, resulting in the accelerated evolution of genomes, and even the emergence of new species. In addition, TEs can also regulate the expression of some important genes in host genome to help the host exhibit some new biological characters via introducing extra regulatory elements, exons and splicing sites.TEs are classified into two major classes based on the types of transposition intermediate: retrotransposons, which have an RNA transposition intermediate, and DNA transposons, which have a DNA transposition intermediate. Tyl-copia superfamily in this study belongs to LTR-retrotransposon subclass. The superfamily harbors several transposition-related encoding regions, as well as one flanking LTR (Long terminal repeat) respectively located on both sides of the element. The transposition of Tyl-copia is mainly controlled by the host regulatory mechanisms and natural selection. So far, the systematic work on Tyl-copia has been mainly studied in plant genomes, and corresponding comprehensive research has been rarely reported in animals or even insects. Moreover, the published data on Tyl-copia was initially produced from traditional PCR and cloning procedures, and in recent years the large-scale genome sequencing has made the identification and studies on these TEs much easier. However, for the non-model organisms such as fig wasps, the large-scale genome sequencing is not feasible because of the high costs. In order to learn the abundance, diversity, distribution and evolution of Tyl-copia elements among fig wasps with a symbiotic life type in the syconium system, we herein isolated and identified nearly160sequences of the gene encoding reverse transcriptase of Tyl-copia elements from genomes of eight fig wasp species associated with Ficus hispida and Ficus benjamina by means of PCR and cloning. The results are as follows:(1) Richness of sequence polymorphism and complexity of distribution pattern exist among the Tyl-copia elements from the eight fig wasp species. The elements, designated as NvTc group, which show high sequence similarity to annotated elements from Nasonia vitripennis, exhibit a high level of abundance in non-pollinating fig wasps, some of which may be able to transpose. However, the NvTc elements from pollinators present contrary scenarios, suggesting that the proportion of the active Tyl-copia in the pollinators’genomes could be very low.(2) Ks/Ka ratios indicate that selective pressures on Tyl-copia from different species of fig wasps, especially between the pollinators and the non-pollinators, are divergent, which are also reflected by the distribution and sequence polymorphism of elements among different wasps. In addition, the polymorphism and evolution mainly depend on the selective pressure of the host genome, which leads to the events of elements loss, though the transposition activity of Tyl-copia should not be neglected.(3) Compared with the phylogenetic tree respectively based on the nuclear sequence ITS2and the mitochondria coxl from the eight fig wasp species, the phylogenetic tree constructed with the Tyl-copia rt is obviously incongruent. Combined with the high similarity among several rt sequences obtained from distantly related species, and the "patchy" distributions of Tyl-copia along the fig wasp lineages, we are inclined to speculate that there have occurred some horizontal transposon transfer (HTT) events among different species of fig wasps. Further experiments are needed to support this hypothesis.