(13) Proposal to designate reproductively competent species of hybrid origin by an × placed in brackets (reword Article H.3 Note 1 )

Interspecific hybridization, a prevalent and problematic aspect of plant diversification, is dealt with in the present Code of Botanical Nomenclature (ICBN, 1988) under the rules for nothotaxa. The majority of interspecific hybrids that form in nature are of isolated and ephemeral occurrence, and are either sexually sterile or otherwise disadvantaged. However, a significant fraction ofnatural hybrids do become reproductively competent and form populations, thus giving rise to species ofhybrid origin that differ significantly from orthospecies that arise through the more usual processes of mutation, recombination, isolation, selection, and drift. Perhaps the most frequent class of taxa ofhybrid origin is that of sexual allopolyploids. However, other classes are known (see below), including stabilized species ofallohomoploid origin. In addition, reproductive competence of hybrids may be effected by various avenues of apomixis, including several modes of vegetative reproduction (see Grant, 1981, for a review of the role of hybridization in plant evolution). For the purposes of the present discussion, the immediate products of interspecific hybridization (i.e., usually sterile F, hybrids) will be referred to as initial nothospecies, and any taxa derived from these that are reproductively competent will be referred to as reproductive nothospecies. Only in certain cases are initial hybrids immediately reproductively competent (fertile allohomoploids and certain types of apomicts particularly). Reproductive nothospecies share most of their features with the initial nothospecies that are their progenitors, but also differ from them in important ways. Both types may be thought of as hybrids, because there is a direct lineage from the initial nothospecies to the reproductive nothospecies. Moreover, the hybrid genotypes of initial nothospecies are usually perpetuated more or less intact by the derivative reproductive nothospecies (cf. Roose and Gottlieb, 1976; Werth et al., 1985a). This is underscored by the contemporaneous occurrence of some allopolyploids with their allodiploid progenitors, whereby their close phenotypic and genotypic similarities are readily apparent. However, in nature, reproductive nothospecies function ecologically much as do the more usual taxa ofdivergent origin by forming populations and becoming permanent and perhaps important components of natural ecosystems. These nothospecies may evolve through mutations (for example allopolyploids may experience silencing ofduplicate genes-Werth et al., 1985b) and thereby acquire novel genotypes (and phenotypes) that are somewhat different from those of their initial nothotaxon ancestors. There is also evidence that reproductive nothotaxa may undergo orthospeciation, thus becoming the nodes from which future clades arise (Werth and Windham, 1990). The present Code provides for indicating hybridity of a taxon by use of a multiplication sign x (H.l.l), used either in a formula (H.2.\.) or placed before the collective specific epithet of the hybrid (or genus name in the case of an intergeneric hybrid) (H.3.\.). A distinction between initial and reproductive nothospecies is implied under Note 1 ofArticle H.3 which reads "Taxa which are believed to be ofhybrid origin need not be designated as nothotaxa." Unfortunately, in many cases this provision presents a dilemma for the treatment of reproductive nothospecies, To designate a reproductive nothospecies as a hybrid by use of the unbracketed x equates it to an initial nothospecies, failing to recognize the intervening evolutionary history ofthe taxon. On the other hand, omitting the x results in a failure to communicate an extremely important attribute of the reproductive nothospecies, i.e., that it is biologically a hybrid, and equates such taxa to those that have arisen through divergent orthospeciation, even though that is not the case. The problem becomes especially acute when both an initial nothotaxon and its derivative reproductive nothotaxon are contemporaneous (and even syntopic), where a choice must be made between treating both as equivalent nothospecies, or treating the reproductive nothospecies as a non-nothotaxon, resulting in confusion. In cases where the distinction between initial and reproductive nothospecies is not clear cut, such as allohomoploids and hybrids that are sexually sterile but reproduce via apomixis, the choice must be made either to treat the taxon as a nothotaxon, or treat it erroneously as an orthospecies with no hybrid history. Thus, the designation of reproductive nothotaxa by the use of the x has been inconsistent. The following proposal is a simple way to alleviate this problem: