| Sumario: | The chloroplast petD gene consists of two exons separated by a group II intron with 6 stem-loop domains (DI–DVI). This study examines the petD intron of taxa representing the major eusporangiate lineages from downloaded GenBank sequences. For the eusporangiate ferns examined here, the intron length varied from 591 to 712 bp. As expected for the 6 group II introns, DI was the largest and accounted, on average, for approximately 52% of the overall intron length, although intron length variation among taxa was observed. Additionally, there did not appear to be a directly proportional relationship between DI and intron lengths. For example, the Equisetum species had the smallest introns (591–597 bp) with a DI (343–345 bp) representing approximately 58% of the overall length. In contrast, the taxa in the genus Ophioglossum all had introns of 712 bp, with their DI (347 bp) making up about 49% of the total length. Length variation between eusporangiate taxa was observed not only for the petD intron itself but also among the domains. Among the eusporangiate ferns, DI did not exhibit the greatest degree of variation (only 324 to 347 bp). Instead, this was observed for DIV (63 to 172 bp). The lowest level of observed length variation was for DV (34 to 35 bp). Also included for comparison were members of the Osmundaceae, a group of ferns considered intermediate between eusporangiate and leptosporangiate ferns and/or as basal leptosporangiate ferns. Intron and domain values for osmundaceous ferns were, in general, comparable with the eusporangiate ferns. Secondary structures were generated, and a phylogenetic analysis was also conducted.
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