Center for Tropical Disease Research and Training, Department of Biological Sciences, University of Notre Dame, Indiana
It has been postulated that because spliceosomal introns are incapable of self-splicing, intron indels should be very rare, if not unique, evolutionary events (Venkatesh, Ning, and Brenner 1999
). This characteristic would make them powerful markers for phylogenetic studies, immune from problems of homoplasy that can affect primary sequence data. Counter to this argument, earlier studies of the white gene (Besansky and Fahey 1997
; Gomulski et al. 2001
) indicated a discouraging degree of promiscuity in the pattern of intron presence or absence in the white sequences then available.
Here we report a more extensive study of the intron-exon organization of the dipteran white, present as a single copy gene in all insects examined to date (Levis, Bingham, and Rubin 1982
; Besansky et al. 1995
; Ke et al. 1997
; Abraham et al. 2000
; Gomulski et al. 2001
; R. Beeman, personal communication). The white gene product, involved in the uptake of pigment precursors into the insect eye, belongs to a large superfamily of ABC transporters that translocate a wide range of substances across cellular membranes in both prokaryotes and eukaryotes (Saurin, Hofnung, and Dassa 1999
). We analyzed an
1-kb gene fragment in 52 insect species from the orders Diptera (48 species), Mecoptera (1 species), Lepidoptera (2 species), Coleoptera (1 species) and homologues from human and Caenorhabditis elegans, identified with a parsimony-based phylogenetic analysis.
Sequences were retrieved from GenBank or were obtained after PCR amplification of genomic DNA using degenerate primers and conditions described earlier (Besansky and Fahey 1997
; Krzywinski, Wilkerson, and Besansky 2001
) (table 1
). We aimed to include representatives of all main dipteran lineages and insect orders closely related to Diptera. However, attempts to extend the sampling by including representatives of the infraorder Bibionomorpha, additional mecopteran taxa, fleas (Siphonaptera) or Strepsiptera, were unsuccessful because of long introns or mismatches (or both) between primers and templates. Apparently, for the same reason, we were able to amplify and sequence only the 5' half of the gene fragment from Panorpa (Mecoptera), Vanessa (Lepidoptera), and Bombyx mori (Lepidoptera).
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The alignment (available on the MBE website) comprised 260 amino acid positions, unambiguously aligned, except for a highly variable region between codons 3971, reported earlier (Besansky and Fahey 1997
). Aligned amino acids guided the nucleotide alignment on which a total of 14 known and predicted intron insertion sites were mapped. These have been numbered according to their location within the alignment, with intron 1 nearest the 5' end (fig. 1
). The unambiguously aligned regions harbor 10 introns, of which four are found in homologous (identical) positions in multiple species. Intron 3 is present exclusively in Diptera, whereas three other dipteran introns (10, 11, and 13) are shared by other insects and C. elegans. The number of introns in individual dipteran species varies from 0 to 4. None of the introns from the human white homologue are shared by other taxa.
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Introns were mapped onto trees inferred in this and earlier studies of Diptera (Yeates and Wiegmann 1999
). Despite the limitations imposed by incompletely resolved branching patterns, it is apparent that presence or absence of introns is not correlated with the phylogenetic relationships among taxa (fig. 1
). The observed distribution suggests relatively frequent and random intron loss during white gene evolution in Diptera. Introns 10, 11, and 13, found in identical positions in insects and C. elegans, must have had a relatively ancient origin, no later than the most recent common ancestor of both lineages (>540 MYA). These introns have been subsequently lost in several dipteran taxa. A reliable inference of the sequence of events and the number of intron losses is hampered by the lack of well-supported phylogenetic relationships within Diptera.
We suggest that of the three introns shared by insects and a roundworm, introns 10 and 11 have been lost three times in distantly related dipteran lineages, whereas intron 13 has been lost four or five times. Loss of intron 10 in a lineage leading to a mosquito Anopheles albitarsis occurred relatively recently, as all the other 28 mosquito species possessed intron 10, including a very close relative, Anopheles albimanus. Of particular interest is an intronless white sequence in a crane fly (Tipulidae). Tipulids are generally regarded as one of the basal clades of Diptera (Yeates and Wiegmann 1999
). Other lineages that originated early in dipteran evolution, including Trichoceridae closely related to Tipulidae, contain two to four introns. It is probable that the most recent common ancestor of Tipulidae and Trichoceridae possessed at least two introns. If so, loss of these introns in a tipulid may have occurred simultaneously, as concerted intron loss appears to be a relatively common phenomenon during genome evolution (Frugoli et al. 1998
).
The most likely mechanism of precise intron deletion is via a spliced or a partially spliced mRNA intermediate which, after reverse transcription, is incorporated into the genome by gene conversion (Lewin 1983
). This mechanism has been demonstrated experimentally in yeast, where replacement of the genomic sequence through homologous recombination with a cDNA copy resulted in intron loss (Derr, Strathen, and Garfinkel 1991
). If processed retrotranscripts are the predominant mediators of intron loss, an important prediction can be made: for an intron loss to be inherited in multicellular organisms, the genes subject to intron loss must be expressed in the germline, in cells developmentally ancestral to the germline, or expressed maternally and supplied to the embryo. Consistent with this prediction, white gene expression has been detected during embryonic development in several insects (Pirrota and Bröckl 1984
; Abraham et al. 2000
; Gomulski et al. 2001
), suggesting that it plays an essential role during early development.
A few convincing examples of intron insertion into new positions have been reported (Logsdon, Stoltzfus, and Doolittle 1998
; Venkatesh, Ning, and Brenner 1999
), but the mechanisms underlying intron gain are poorly understood. Likely mechanisms include insertion by transposable elements, as found in maize (Giroux et al. 1994
); reverse splicing of a spliceosomal intron into a new site, followed by retrotranscript-mediated recombination; and tandem duplication of exons, if the duplicated sequence contains or acquires the elements necessary for splicing (Logsdon, Stoltzfus, and Doolittle 1998
; Venkatesh, Ning, and Brenner 1999
).
The exclusive presence of intron 3 in Diptera suggests its recent gain. The intron is found in a lower dipteran Agathon elegantulus (Nematocera, Blephariceridae) and in most taxa of the higher flies from the infraorder Muscomorpha. Such a distribution is most parsimoniously explained by a single intron gain in a common ancestor of Blephariceridae and higher flies, followed by loss in at least two lineages. Uncertainty surrounding the presumed insertion event and its timing will remain problematic, pending improved resolution of basal lineages of Diptera and extended taxon sampling. In particular, more taxa representing basal dipteran lineages and lineages that diverged shortly before the emergence of Diptera are necessary to increase confidence in the timing of the intron acquisition. Note, however, that the assignment of introns as newly gained must be tentative (Bhattacharya and Weber 1997
). On the basis of six sequences, Gomulski et al. (2001)
inferred an increase in the number of the white gene introns from the lower Diptera (Culicidae) to higher Diptera (Calliphoridae and Tephritidae) with an intermediate state in Drosophila melanogaster. However, extended sampling in the present study revealed that two introns, putatively acquired by higher flies (Gomulski et al. 2001
), were actually present in a lower fly A. elegantulus, in nondipteran insects, and in a roundworm. This example emphasizes the difficulty of reconstructing the evolution of gene structure because of a labile nature of intron presence or absence.
Recent advances in automation of molecular techniques have resulted in the accumulation of tremendous amounts of sequence data. The postgenomic era opens the opportunity to utilize in phylogenetics not only sequences of numerous genes but also other types of information embedded in DNA sequences, including intron indels, retroposon integrations, or changes in organelle gene order. In two recent studies, intron indels have been used for tracing monophyletic lineages and establishing evolutionary relationships. Rokas, Kathirithamby, and Holland (1999)
used information about intron insertion in the engrailed homeobox gene of Diptera and Lepidoptera and intron absence in corresponding gene positions of Strepsiptera and other taxa as evidence against a close affiliation between Diptera and Strepsiptera. Venkatesh, Ning, and Brenner (1999)
used intron indels in six genes to define lineages of actinopterygian fishes and to resolve relationships among them. In common with the classical use of chromosomal inversions to reconstruct phylogeny, the use of intron indels is premised on the assumption that their occurrence at a given position is exceedingly rare, if not unique, thereby providing enormous potential for molecular systematics (Rokas and Holland 2000
). Our study, however, demonstrates that this criterion may not be met, and that intron presence or absence may not be a reliable source of phylogenetic information. The white gene intron indels in Diptera show a high level of homoplasy caused by multiple independent intron losses in distantly related lineages. Other genes not only in insects but also in other taxa may show a similar extent of homoplasy. We believe that intron indels can offer valuable insights into evolutionary history, particularly in relatively recent taxa or taxa conservative with regard to intron differences in their genes, such as vertebrates (Logsdon, Stoltzfus, and Doolittle 1998
). However, caution should be exercised in their use as phylogenetic characters at deeper taxonomic levels, particularly within insects.
Acknowledgements
The authors thank J. Johnson, W. Turner, H. Pratt, U. Willhoeft, M. Goldsmith, K. Okano, C. Porter, and R. Vargas for generously supplying specimens and B. Wiegmann for helpful comments on an earlier draft. R. Beeman kindly shared data prior to publication. J. Bedell and T. Fahey provided skilled technical assistance. Support from the UNDP/World Bank/WHO Special Programme for Research and Training in Tropical Diseases to N.J.B. is gratefully acknowledged.
Footnotes
Keywords: spliceosomal introns
intron loss
white gene
phylogeny
Diptera
Address for correspondence and reprints: Jaroslaw Krzywinski, Center for Tropical Disease Research and Training, Department of Biological Sciences, University of Notre Dame, P.O. Box 369, Notre Dame, Indiana 46556-0369. jkrzywin{at}nd.edu
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