With even more distant comparison to the eudicots, >220 million years of independent descent, only rare segments of genic colinearity are observed at either full genome or local genome scales [58].
In three of the four comparisons (human with tarsier, tree shrew, and opossum), significant bias was still identifiable between human and the more distant comparison species (supplementary table S1, Supplementary Material online).
In more distant comparisons, with longer ancestral divergence times, colinearity across orthologous regions appears to be much more rare than within the grasses.
Given the early availability of both the human and mouse genomes, initial efforts focused on human mouse pair-wise comparisons; but subsequent studies frequently used more distant comparisons such as human-fish to uncover functional non-coding elements with a higher stringency [15][16].
We may thus expect estimates of selective constraint to be lower for more distant comparisons.
We conducted analyses with and without these more distant comparisons (for details see supplementary material, Supplementary Material online).
As in other distance measures, this finds the most similar sequence fragments and is biased high, particularly for more distant comparisons.
More distant comparisons with mosquitoes, moths, beetles, and bees reveal hugely expanded and contracted lineages over time frames of 250 300 My (million years) (see, for example, [ 27]).
This illustrates that the read mapping utility should be restricted to the analysis of strains from the same species, with BLAST being the preferable option for more distant comparisons.
Close genomic relationships have also been observed for more distant comparisons, such as between Glycine and Medicago [ 7], for which substantial regions of almost perfect colinearity have been observed.
Therefore, the compilation of a basal insect genomic resource, such as the one presented here, will facilitate genomic comparisons across 350 million years of insect diversification, and will serve as a phylogenetic link to even more distant comparisons, such as crustaceans (e.g. Daphnia) and chelicerates (e.g. tick), and beyond.
