HYBRIDISATION BETWEEN DIPLOID AND TETRAPLOID 

 SPECIES OF EUPHRASIA 



By P. F. Yeo 



University Botanic Garden, Cambridge* 



The chromosome numbers of the British species of Euphrasia have been tabulated 

 in a previous paper (Yeo, 1954). Five species are diploid with n = 11. They are E. 

 rostkoviana Hayne, E. montana Jord., E. rivularis Pugsl., E. anglica Pugsl. and E. hirtella 

 Jord. These constitute the series Hirtellae, characterized by the possession of long 

 glandular hairs, which in Britain only occur' exceptionally outside the Hirtellae. In 

 other groups, only the tetraploid number, n = 22, has been found, and it is probable 

 that this is the chromosome number of all the other British species. 



Warburg (1952) lists four hybrids whose parents are now known to be diploid and 

 tetraploid respectively. Such hybrids would be expected to be sterile triploids and to 

 occur only as occasional isolated individuals. However, in at least some cases, they are 

 fertile and form populations of more or less numerous plants. Several populations of 

 this type, representing various forms, have now been investigated. Observations made 

 at Withypool, S. Somerset, will be reported first. 



Diploid-Tetraploid Hybrids at Withypool 



The plants concerned were found on a heath at 1200 feet elevation near Withypool, 

 Exmoor (grid. ref. 21/826367), in August, 1952. 



Four forms of Euphrasia were present; their characters and habitats are given in 

 Table 1, and herbarium specimens are shown in plate 17, fig. 1. 



The flower measurements for E. anglica given in the table represent a population 

 (E.180) about a mile nearer Withypool than that (E.187B) which accompanied the 

 hybrids. The specimens in the photograph were also taken from this population since 

 the plants of E.187B show slight traces of the hybrid influence which is present in E.187A. 

 Owing to their occurring in grazed turf, it is difficult to find intact plants of E. anglica 

 late in the season, and neither of the two plants in the photograph is intact; that on the 

 left has been drawn up by long grass but has been included because it shows the leaves 

 relatively well. 



It will be seen from Table 1 and Plate 17, fig. 1, that there were two species present, 

 E. anglica and E. micrantha Rchb., which are widely different morphologically, and 

 which differ in habitat. The dwarf habit of E. anglica is correlated with its short-turf 

 habitat, while the tall E. micrantha is found in the immediate vicinity of Calluna vulgaris 

 and Erica cinerea. In addition there was a single triploid hybrid, very like E. micrantha 

 but with a slightly larger flower, and some stalked glands, which, together with the chromo- 

 some number, indicate E. anglica as the other parent. Finally, there was a population 

 of a diploid form which is rather variable, but which possesses some of the characters 

 of E. anglica and some of E. micrantha. In the characters listed in Table 1 it resembles 

 E. anglica in four, E. micrantha in four, and is intermediate in three. It also differs from 

 E. anglica in growing in taller grass. In its extreme form (No. 3 in Plate 17, fig. 1) it is 

 very similar to the triploid. The morphological distinctness of this form would qualify 



• The work reported in this paper was carried out during the tenure of a Research Scholarship at the University College of 

 Leicester. 



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