Attempts to molecularly distinguish cryptic taxa in Anopheles gambiae s.s.

Abstract

Analyses of inversions in polytene chromosomes indicate that, in West Africa, Anopheles gambiae (sensu stricto) may be a complex of more than a single taxonomic unit, and these units have been called chromosomal forms. In order to determine whether this genetic discontinuity extends to the rest of the genome, as would be expected if reproductive isolation exists, we have sequenced several regions of both the nuclear and mitochondrial genomes. With one exception, we were unable to identify any nucleotide sites that differentiate the chromosomal forms. The exception was the internal transcribed spacer (ITS) of the ribosomal DNA (rDNA). Three sites in this region distinguish Mopti chromosomal form from Savanna and Bamako in Mali and Burkina Faso. However, outside these two countries, the association between chromosomal form and rDNA type does not always hold. Together with the variants in the rDNA intergenic spacer (IGS) described in the accompanying papers (della Torre et al., 2001; Favia et al., 2001), we can recognize two major types of rDNA, Type I and Type II (corresponding to molecular forms S and M in della Torre et al., 2001). Type I is widespread in West Africa and is the only type found outside of West Africa (i.e. Tanzania and Madagascar). Type II is confined to West Africa. We were unable to detect any heterozygosity for the ITS types even in five collections containing both types. A sample from the island of São Tomé could not be classified into either Type I or Type II as the rDNA had characteristics of both. In general, our results confirm that An. gambiae is not a single pan-mictic unit, but exactly how to define any new taxa remains problematic. Finally, we have found minor variants of the major rDNA types fixed in local populations; contrary to most previous studies, this suggests restricted gene flow among populations of this species.

Ecological Zones Rather Than Molecular Forms Predict Genetic Differentiation in the Malaria VectorAnopheles gambiaes.s. in Ghana

“…The other loci (including the rDNA) are at the proximal (centromeric) end of the X chromosome Lehmann et al 2003). However, little differentiation has been found at loci elsewhere in the genome (Gentile et al 2001;Mukabayire et al 2001;Wang et al 2001;Turner et al 2005). Debate is ongoing as to whether such generally limited differentiation primarily reflects insufficient time for differentiation to have occurred or ongoing introgression among forms.…”

“…The lack of marked differentiation throughout most of the genome has been attributed to the recent isolation of the forms whereby insufficient time has elapsed for even noncoding DNA to accrue differences (Gentile et al 2001). Premating isolation mechanisms apparently result in positive assortative mating when the forms are in sympatry, with 98.8% within form mating (Tripet et al 2001).…”

Revisiting the role of introgression vs shared ancestral polymorphisms as key processes shaping genetic diversity in the recently separated sibling species of the Anopheles gambiae complex

“…These data are in accordance with the studies of Wondji et al (2002) and Lehmann et al (2003) that provided evidence for a degree of genetic isolation between M and S forms. The failure of other studies to detect differences between M and S forms reflects how recent the disruption of gene flow must have been (Gentile et al, 2001) or conversely that ongoing gene flow may be homogenising allele arrays in both forms, outside certain regions of the genome. This study utilised haplotype frequency-based approaches, Figure 3 Statistical parsimony network for A. arabiensis data set.…”

Molecular detection ofCulicoidesspp. andCulicoides imicola, the principal vector of bluetongue (BT) and African horse sickness (AHS) in Africa and Europe

“…nili and An. minimus using ITS) [9,10,13,22,26]. In another study the ITS1 sequences of C. arakawai, and eight other species, were used to produce an initial partial phylogeny of the species within the genus [15].…”

“…nili and An. minimus using ITS) [9,10,13,22,26]. The alignment of the 11 ITS1 sequences of Culicoides obtained from Genbank and EMBL databases helped us to focus on one region in the 5' end and one in the 3' end appeared highly conserved.…”