Structure of the ECB Δ11 desaturase gene, primer locations, and regions amplified. (a) The complete Δ11 desaturase gene, showing primer locations. Exons are shown as gray bars, introns as black lines, primers as arrows. Numbers below the intron/exon boundaries indicate positions of intron start and end points in the Δ11 desaturase coding sequence. (b) Region sequenced for genealogical analysis. The region between base pairs 415-771 of the coding region contains 357 bp of exon (11 bp exon 1, 338 bp exon 2, 8 bp exon 3) and 1008-1988 bp of intron (105-279 bp intron 1, 903-1883 bp intron 2). (c) Size of expected products from clade specific PCR. Four PCR products of different lengths are shown. Each results from a PCR reaction using the specified primer pairs. For each primer pair, one primer matches sequence only from clade A or clade B. Locations of the six indels responsible for the 87 bp length difference between clades are indicated with vertical dashes.