Detecting Abortive Spores in Herbarium Specimens of Sterile Hybrids

Abstract

Uses of herbarium specimens for biosystematic research are being extended by new techniques, and those described here have been exploited only in the past few decades. Not only do dried and pressed pteridophyte specimens provide such classical gross data as geographical, ecological, and morphological, but microscopic and chemical data as well. Specimens on herbarium sheets often yield hypotheses of hybridity by morphology alone, based on the phenomenon of intermediacy (W. Wagner, 1983), in which the characters of higher plant hybrids tend to have expressions lying between those of the parental species: If one has ovate leaves, and the other lanceolate leaves, the hybrid has more or less ovate-lanceolate leaves. The hybrid's characters are not necessarily in the middle; some may be closer to one parent, some closer to the other. Probably most investigations of hybrid pteridophytes begin in the herbarium, where specimens showing interspecific intermediacy are encountered. We call such specimens because they do not fit either presumed parent. Whenever they are found we examine their spores. Interspecific pteridophyte hybrids fall into three categories as regards spore production, namely (a) sterile hybrids that have defective spores (e.g., Fig. 1), (b) fertile hybrids (either introgressant or, much more commonly, allopolyploid) that have normal viable spores, and (c) apogamous hybrids that produce unreduced spores. We are not concerned here with fertile hybrids, except for those that are apogamous (e.g., in the United States, Pteris x hillebrandii and Asplenium x heteroresiliens). Obligately apogamous hybrids can usually be recognized because some or most of the sporangia have only 32 spores that are viable; those with 64 spores show abortion (various combinations are also possible; cf. Manton, 1950). Apogamous intertaxa involve one apogamous and one sexual parent. Non-hybrid taxa with normally 32 or 16 spores per sporangium might be confused with apogamous hybrids but these can be recognized because the reduced spore number is regularized, characteristic of all sporangia, and the spores are non-abortive. Examples of 32- or 16-spored sporangia in normal sexual species are found in a wide diversity of genera (e.g., Ceratopteris, Cheilanthes, Lindsaea, Alsophila, Sadleria). Allopolyploid sexual hybrids behave like ordinary species, even though they are actually hybrids, and many of these are frequent or common; they are recognized by their spore size, usually larger than that of their parents. Not all spore abortion is associated with hybridity (see below under Additional Perspectives). Nearly all spore samples from normal species show at least some deviant spores. It is not unusual to find as much as 10-20 percent of spores