- Research article
- Open Access
Polytene chromosomes as indicators of phylogeny in several species groups of Drosophila
© O'Grady et al; licensee BioMed Central Ltd. 2001
- Received: 17 May 2001
- Accepted: 10 October 2001
- Published: 10 October 2001
Polytene chromosome banding patterns have long been used by Drosophila evolutionists to infer degree of relatedness among taxa. Recently, nucleotide sequences have preempted this traditional method. We place the classical Drosophila evolutionary biology tools of polytene chromosome inversion analysis in a phylogenetic context and assess their utility in comparison to nucleotide sequences.
A simultaneous analysis framework was used to examine the congruence of the chromosomal inversion data with more recent DNA sequence data in four Drosophila species groups – the melanogaster, virilis, repleta, and picture wing. Inversions and nucleotides were highly congruent with one another based on incongruence length difference and partitioned Bremer support values. Inversion phylogenies were less resolved because of fewer numbers of characters. Partitioned Bremer supports, corrected for the number of characters in each matrix, were higher for inversion matrices.
Polytene chromosome data are highly congruent with DNA sequence data and, when placed in a simultaneous analysis framework, are shown to be more information rich than nucleotide data.
- Consistency Index
- Polytene Chromosome
- Chromosomal Inversion
- Inversion Data
- Decay Index
Species in the family Drosophilidae have been premier research subjects in evolutionary biology since T. H. Morgan first used Drosophila melanogaster as a genetic tool in the early part of the 20th Century. Polytene chromosome phylogenies have become commonplace in the examination of this family of flies. The chromosomal analyses have been used in two ways in evolutionary studies. The first use is as genetic markers [1, 2] in which the chromosomal inversions are considered alleles and are utilized to examine gene flow and other population genetic parameters. The second use of polytene chromosomes in evolutionary studies is as tracers of phylogeny [3–7]. This approach has resulted in important and detailed chromosomal phylogenies for several groups of flies in the genus Drosophila. Lists of species for which polytene chromosome maps have been produced are available [8, 9], and over 300 species of Drosophila have been examined at this level. In particular, extensive chromosomal phylogenies for flies in the picture wing,melanogaster, virilis, and repleta species groups exist. Cladistic analyses of the chromosomal data for these groups exist for the picture wing and melanogaster species groups . More recently, large amounts of DNA sequence information have been collected for these and many other species groups, yet no detailed analysis of the overall utility of chromosomal inversion data or of their congruence with DNA sequence data has been carried out.
The main objective of the present study is to place the chromosomal inversion data into a phylogenetic framework. We examine the congruence of DNA sequence characters and chromosomal inversion data in four different species groups in the genus Drosophila (picture wing, repleta, melanogaster and virilis) to assess the relative contribution to phylogenetic hypotheses that the two different sources of character information make.
Data Sources for the five data matrices used in this study.
Tree statistics for the four analyses.
picture wing group
Results of partitioning Bremer support to molecular and inversion character sources.
corrected total BS
% zero or
Classical Drosophila studies have used chromosomal inversions to understand phylogeny and speciation. More recent DNA sequence information can be combined with these classical data to make inferences about phylogeny and species divergence. The approach we have taken here is to combine the chromosomal inversion data with DNA sequence data to examine some of the classical notions of Drosophila evolution. Our results suggest that there is a great deal of congruence among DNA sequence data and chromosomal inversion data. Although this result is reassuring, there are still some areas of the phylogenies of the four species groups examined here that are not congruent. These areas are indicative of poor phylogenetic signal from one or both of the kinds of data – DNA and chromosomal inversion data. Chromosomal inversion data are much more information rich as assessed in the present study, and this is probably due to the higher consistency of these characters. This higher consistency stems from that fact that, while nucleotide characters have only four possible states to change to and from, chromosome breaks can occur at many different locations. Therefore, it is much simpler to change from an A to a T in two unrelated lineages than it is to have the exact same chromosome break in those same lineages.
Four data matrices were constructed using DNA sequences and chromosomal inversion data from the literature (Table 1). The four species groups for which we have obtained inversion data represent the four major species groups for which such data exist. Chromosome inversion information for other smaller groups such as the antopocerus species group (Hawaiian Drosophila) is published , however DNA sequence data are not yet available for these groups.
Phylogenetic trees were generated using PAUP 4.0b7 . In order to assess the relative contribution of chromosomal inversion and DNA sequence data we placed our analysis in a simultaneous analysis framework [15–17]. Bootstrap values were computed using PAUP 4.0b7. Decay indices were computed using AUTODECAY  and using the methods described in Baker et al. . Significance of Incongruence Length Differences (ILDs [20, 21]) were calculated using the Partition Homogeneity Option in PAUP 4.0b7 . Partitioned Bremer supports for the inversion partition and the DNA partition were calculated using the approaches outlined in Baker et al. .
Here we include definitions of several phylogenetic measures used in this paper. The consistency index (CI; ) is used to determine how much homoplasy (i.e., how many times a character evolves on a tree) a given character displays. The CI of a character is the mimimum number of steps for that character on a given tree divided by the total number of steps reconstructed for that character on the same tree. Those characters which are highly consistent, or without homoplasy, would have a consistency index of 1.
The decay index or Bremer support value is used to measure support at a node of interest in a phylogenetic tree. To obtain the decay index, the treelength of a tree constrained not to contain a node of interest is substracted from the unconstrained (most parsimonious) treelength. Higher numbers generally indicate greater support at a node. Partitioned Bremer support (PBS; ) measures the amount of support provided by each individual partition to the DI for every node in the combined analysis phylogenies. PBS is an extension of the decay index in that it shows the contribution of each partition to the decay index of every node on the combined analysis tree. To obtain the PBS value for a given node on a combined tree, the length of the partition on the unconstrained combined tree is subtracted from the length of a partition on a tree constrained to not contain the node of interest. If the partition supports a relationship represented by a node in the combined tree, then the PBS value will be positive. If, on the other hand, a partition supports an alternative relationship, the PBS value will be negative. The magnitude of PBS values indicate the level of support for, or disagreement with, a node. The sum of all partition lengths for any given node will always equal the decay index for that node on the total evidence tree.
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