Parentucellia is a small genus with only two species in western Europe, the Mediterranean and southwestern Asia. Both are more or less weedy and have been recorded far beyond their native distribution range, also in Belgium.
Molecular data (Bennett & Mathews 2006) suggest that Parentucellia is in fact nested in Bartsia. Bartsia trixago L. and Parentucellia viscosa (L.) Caruel were shown in a polytomy, while the second species of Parentucellia, P. latifolia (L.) Caruel, appears as sister to the South American species of Bartsia (Scheunert & al. 2012). Additional studies, however, have shown that it is best kept in Parentucellia (Uribe-Convers & Tank 2016).
1 Corolla yellow, 16-24 mm long, caducous. Calyx teeth ca. as long as tube. Ovary and capsule hairy at apex === Parentucellia viscosa
Corolla reddish-purple (rarely white), much smaller (8-10 mm long), persisting. Calyx teeth only ½ as long as tube. Ovary and capsule glabrous == P. latifolia
Bennet J.R. & Mathews S. (2006) Phylogeny of the parasitic plant family Orobanchaceae inferred from phytochrome A. American Journal of Botany 93(7): 1039–1051. [available online at: http://www2.huh.harvard.edu/research/mathews-lab/LabWebsite/pdfs/2006ajb.pdf]
Scheunert A., Fleischmann A., Olano-Marín C., Bräuchler C. & Heubl G. (2012) Phylogeny of tribe Rhinantheae (Orobanchaceae) with a focus on biogeography, cytology and re-examination of generic concepts. Taxon 61(6): 1269-1285. [available online at: https://www.researchgate.net/publication/281691099_Phylogeny_of_tribe_Rh...
Uribe-Convers S. & Tank D.C. (2016) Phylogenetic Revision of the genus Bartsia (Orobanchaceae), disjunct distributions correlate to independent lineages. Systematic Botany 41(3): 672-684.