THE ANNOTATION OF INTROGRESSANTS

The recent paper by Hui-Lin Li (Taxon 6: 216-218. 1957) brought up a problem about which a number of us dealing with introgression have been wondering, namely what is the best method for designating backcrosses and members of hybrid swarms. In a paper dealing with introgression in Aesculus (Rhodora 59: 185-203. 1957) I used a different formula from the one proposed by Li, but it was stated then that there may be a more satisfactory method. It seems generally agreed that there should be a formula of precise meaning which would convey the investigator's concept of a plant or population which is of hybrid origin. The problem arises in finding a suitable formula which indicates the nature of a specimen, and which could be used as an annotation on the herbarium sheet. This is important not only because of the biological significance of hybridization, or for a greater precision in nomenclature, but also important to the herbarium curator facing the problem of filing the specimens. During the fall of 1956 I took the liberty to write to a number of taxonomists who I thought would have dealt in some degree with this problem. The information gained from this correspondence was that there were many different methods of annotation currently being used. It seems fitting at this time, after Li's paper, to describe briefly the methods of annotation already in use, and some thoughts bearing on the problem. A number of investigators have used a formula such as "species A" (X "species B") to indicate that the specimen is most nearly referable to "species A" on the basis of all characters. This gives the herbarium curator an opportunity to file the specimen under that species. The name of the parent which apparently contributed the gene minority is placed in parentheses, or brackets, and preceded by the multiplication sign "X." This is the method which I used for the annotation of hybrid forms in Aesculus, e.g., A. sylvatica (X pavia). There is objection, however, to this method, for this may possibly be interpreted as indicating that the investigator thought the specimen might represent an F1 hybrid but that he was not too sure. Also one might possibly think that (X pavia) was meant for a synonym instead of an explanation. Because this formula may be ambiguous and confusing, it would be the obligation of the investigator to make known the meaning of his method of annotation. Others have used the symbols > and (greater than) and keep the specific epithets in alphabetical order, following Art. H 1