The notion that interspecific hybrids are rare is ill-founded. The only modern broad compilations of their numbers (i.e. number of species combinations) are those of Knobloch,248 249,250 who recorded 23 675 in the angiosperms and gymnosperms and 620 in the pteridophytes. However, this was an uncritical survey, many fictitious hybrids being included, as were those only artificially produced. In terms of well-substantiated, naturally occurring hybrids the figure is much lower, but there must be a great many undetected hybrids in underworked areas, especially the tropics. Realistic figures can be obtained only from very well studied floras, such as that of the British Isles. Stace405 recorded 626 ‘well substantiated interspecific’ hybrids among vascular plants, plus 122 possibly correctly identified ones (as well as 227 errors) in the British Isles; since that compilation at least 100 additional combinations have been discovered. Taking into account the doubtful cases above, and a few errors in
the 626, it iN reasonable to assume that there are about 780 interspecific hybrids among the British vascular plant flora of about 2 500 native and alien Nexual species (i.e. excluding apomictiq taxa), ignoring the many combina tions recorded from only outside the British Isles yet involving two British species. The figure of 2 500 represents, very approximately, 1% of the world’s vascular flora, which might therefore include in the region of 78 000 different naturally occurring interspecific hybrids. Although most of these are uncom mon, clearly hybridization is not a rare or abnormal phenomenon. Moreover, a great many firmly accepted species are known to be of hybrid origin. According to the levels of polyploidy considered to be exhibited by angio- sperms, this applies to between 30 and 70% of species. When one considers the potential for hybridization, hybrids assume even greater importance. There are vastly more artificial hybrids known than natural ones; in the Orchidaceae alone about 75 000 have been synthesized. Indeed, one is forced to the conclusion that in general the ability to hybridize is the usual situation.343 In the British flora about half the genera are monotypic, and about half the rest form at least one hybrid in the wild.
A small proportion of naturally occurring hybrids involves three or even more species. This, of course, is always a possibility when a primary (dispecific) hybrid is fertile, since it might then mate with a third species. About 31 such hybrids have been recorded from the British Isles. Again, the possibilities are much greater in the case of artificial hybridizations, and a hybrid incorporating 13 different parental species of Salix has been synthe sized in Sweden.310
In the non-vascular groups much less is known concerning natural hybri dization. In terms of recorded numbers hybrids are much less common than in the vascular plants, but probably most have been overlooked. In the bryophytes they are usually sterile, so they exist only as hybrid sporophytes upon a female gametophyte and are relatively difficult to detect. The fact that they have been recorded in a quite wide range of bryophytes, and also of algae, suggests that hybridization in an important phenomenon in all groups of plants (as well as in fungi).
Intergeneric hybrids are much less common. In the British vascular plant flora there are only 34 interspecific combinations of intergeneric hybrids known, involving only 17 intergeneric combinations. Extrapolated to a world scale, this indicates that there might be about 2 930 intergeneric hybrid combinations. But again, there are many more artificially produced in tergeneric hybrids, and natural intergeneric hybrids occur in algae and bryophytes as well as in the vascular plants. ‘Wide’ hybridization is particu larly a feature of the Orchidaceae, where artificial hybrids involving as many as eight different genera in one offspring have been synthesized. In the Poaceae, another family in which intergeneric hybrids are rather frequent (although here, unlike the Orchidaceae, they are mostly sterile), all naturally occurring hybrids are between genera within one tribe, although intertribal crosses have been artificially synthesized. On the other hand no hybrids have so far been discovered or synthesized between plant species assigned to different families.
Little generalization can be made concerning the frequency of hybridiza tion, since between extreme cases there is every intermediate. At one
The recognition o f hybrids 133
extreme one may cite the case of the grass Catapodium marinum x
C. rigidum, known only as a single plant found in 1960 in Wales.30 Both
parents and the hybrid are annuals, so that this individual lived for only a few months. At the other extreme come hybrids such as Geum rivale X
G. urbanum, Betula pendula X B. pubescens and Circaea alpina X
C. lutetiana, which appear to occur whenever the two parents come into contact, and which in many regions have become more common than one or even both parents. Naturally, the majority of hybrid combinations come somewhere between these extremes, and most nearer the former situation, but there are certainly a great many very common and widespread hybrids.
The prospects of artificial hybridization at one time seemed vastly greater when the technique of somatic hybridization was introduced. This involves the fusing of single cells from two species in cell-culture, and the raising of a hybrid plant from this fusion product. If the two cells were diploid the resultant hybrid is already an amphidiploid, but if haploid cells (e.g. from anther-culture) are used a diploid hybrid results. Despite the early promise of the technique, since the production of the first mature somatic hybrid in 1972 only 27 further such hybrids have been obtained. Moreover 26 of these 28 successful experiments involve Solanaceae, and 18 of them are within the genus Nicotiana, 121 The other two somatic hybrids are within Daucus
,
and todate no intergeneric combinations have been grown to maturity. However, wider crosses might be attainable with improved techniques.