MIMULUS IN GREAT BRITAIN- A CYTOTAXONOMIC NOTE

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1 New Phytol (1975) 74, 155-^6. MIMULUS IN GREAT BRITAIN- A CYTOTAXONOMIC NOTE BY P. F. PARKER Botanical Laboratories, The University, Leicester LEi jrh {Received 4 November 1973) SUMMARY The chromosome numbers and external morphology of Mimulus naturalized in Britain are compared with those of experimentally produced Fi hybrids between Mimulus guttatus DC. and M. luteus L. INTRODUCTION Mimulus guttatus DC. is a native of western North America, its range extending from California to Alaska. Introduced into Britain about 1812, it was recorded in 183 as an established garden escape at Downton, colonizing the wet meadows and river banks ofthe Wiltshire Avon. Salisbury (1964) gives the full history of the spread of the species. M. luteus L., a South American species from Chile, is less common than M. guttatus and has been recorded mainly from northern England and Scotland. Although both species are known to be present, most natural British populations have been reported as sterile hybrids between the two (Salisbury, 1964; Roberts, 1964). Chromosome counts of these two species in the New World have been made mainly by Vickery and his co-workers (Vickery, 1955; Mukherjee and Vickery, 1959; Mukherjee and Vickery, 1962; McArthur et al, 1972). Most American counts of M. guttatus have given 2n = with a small proportion of 2n = 56 also being reported. For Britain there is only one published count of 2«= 48 (Maude, 1939). M. luteus is recorded in America as having chromosome numbers of zn = 6, 62 or 64 (Mukherjee and Vickery, 1962). Successful hybridization of the two species was reported by Mukherjee and Vickery (1962); they crossed diploid M. guttatus with M. luteus plants of all three chromosome races: correspondingly the hybrid progeny were 2n =, 45 or. These hybrids were infertile, and cytological analysis showed that this was due to a high proportion of univalents being formed at meiosis. The objective of the present work was to make a preliminary investigation of the chromosome numbers of naturalized British plants in order to gain some idea of the range of chromosome numbers present. MATERIALS AND METHODS Living material was collected from thirteen sites (Table i). Where a very large continuous area was covered as in Monsal Dale, or separate plants were very common as in the River Lathkill, more than one sample was taken, in order to check whether one or several clones were present in the area. Both Mimulus guttatus and M. luteus were grown trom 155

2 156 P. F. PARKER Botanic Garden stock seeds, and after identification w^ere used both as reference specimens and as parents of artificially produced Fi hybrids which were also grown for morphological comparison with the naturalized plants. All samples were grown in a glasshouse in a i6-h photoperiod during the late autumn and winter; for cytological work, root-tips were pre-treated in a saturated solution of a-bromo-naphthalene for 2 h, then fixed in Carnoy solution. Root-tips were hydrolysed and stained in a mixture of 2% propionic orcein and N HCl (9:1) for approximately 25 min then mounted and squashed in i % propionic orcein. The chromosomes oi Mimulus are small. Counts were made from at least two root-tips from each plant, the cells analysed were drawn, and many were photographed. Pollen fertilities of the clones were estimated by squashing mature, unopened anthers in acetocarmine and counting at least 3 grains from each of three flowers. Leaf and flower morphology of the plants were compared by taking a representative vegetative leaf and a representative flower from each specimen (Figs, i and 2). Table i. Localities of naturalized Mimulus, and sources of reference collections of M. guttatus DC and M. luteus L. County Wiltshire Radnorshire Cardiganshire Merioneth Caernarvonshire Leieestershire Derbyshire Wigtownshire Botanic Garden(s) Uppsala, Bratislava, Cluj, Uppsala Locality South Newton Codford St Mary New Radnor Yspyty Cynfyn Trawsfynnydd Harlech Nr Llanengan, Lleyn Peninsula Coalville Blaekbrook River Lathkill Bowers Hall Monsal Dale Galloway Leningrad Grid. SU ST SO SN SH SH SH 2273 SK SK 5172 SK SK SK NX 396 ref. and habitat Banks of River Wylie Banks of River Wylie Bank of small stream Small stream Diteh Wet ground below Harleeh Castle Banks of R. Soch (2 collections) Mud bank in stream By Reservoir on Marshy ground Isolated rocks in middle of R. Lathkill (4 eollections) In Grassland (2 collections) River Wye, shallow. gravelly area (2 collections) Wet ditch near cottage Referenee Species M. guttatus M. luteus OBSERVATIONS AND DISCUSSION Cytology and pollen fertility. Table 2 shows the chromosome counts and pollen fertilities of the naturalized plants studied, the reference collections, and two Fi hybrid progenies. Only three naturalized plant collections had the diploid number of Mimulus guttatus. The remainder, with the exception of the Llanengan sample, have chromosome numbers of the same order as experimental Fi hybrids from the cross M. guttatus x M. luteus. These numbers also show that at least two of the three known cytotypes of M. luteus have been present in the British flora in recent times. The pollen fertilities of these naturalized plants with intermediate chromosome numbers are quite low (-26%). In comparison, the naturalized diploid plants had fertilities of 57-94%, and the reference diploids 94-98%. The two cytotypes of M. luteus had a fertility range of 59-1%.

3 Mimulus in Britain 157 The Llanengan samples extend our knowledge of Mimulus in Britain. Up to the present time, it has been assumed (Mukherjee and Vickery, 1962; Roberts, 1964) that hybrids between M. guttatus and M. luteus would be unable to produce progeny, but this does not appear to be the case. Tai and Vickery (1972) have shown that in microsporogenesis in M. glabratus (2w = 3), both intra-varietal population hybrids and intervarietal hybrids show quite a high level of unequal chromosome disjunction (7% and 5% respectively) giving viable aneuploid pollen grains with higher as well as lower chromosome numbers. Spontaneous polyploidization also occurs (1.5% and 3% respectively), A similar situation appears to hold in M. guttatus^ where at the southern limit of its range. Table 2. Chromosome numhers and pollen fertilities {estimated hy a staining technique) of naturalized plants, experimental Mimulus hyhrids, and M. guttatus and M. luteus Origin of Material Natural collections S. Newton Codford St Mary New Radnor Yspyty Cynfyn Trawsfynnydd Harlech Llanengan i. Llanengan 2. Coalville Blackbrook River Lathkill r. River Lathkill 4. Bowers Hall i. Bowers Hall 2. Monsal Dale i. Monsal Dale 2. Galloway Mimulus guttatus Uppsala, Bratislava, Leningrad (5 plants each) Mimulus luteus Cluj (5 plants) Uppsala (5 plants) M. guttatus X M. luteus Fi hybrids (Uppsala) Cross I Plant i Cross I Plant 2 Cross IV Plant i Cross IV Plant 2 Number Pollen fertility (% I J * II 6 I I 2 * All four plants were checked for pollen fertility. No. 2 had %, No. 3 had 8%. zn == 56, auto-polyploid plants occur, either growing with diploids or as separate populations, whereas at the northern limit, separate, morphologically more distinct tetraploid populations are formed (McArthur et al., 1972). The chromosome complement of the Llanengan plants {zn = 54) could well have arisen by a rare successful fertilization of a diploid plant with fertile high aneuploid pollen from an Fi hybrid. Morphology. The morphology of the wild samples was of interest as they generally resembled M. guttatus rather closely, unlike the material described by Roberts (1964), which appeared to be more variable in flower patterning. Line drawings of a typical flower and leaf from each wild sample are presented in Fig. i, and in Fig. 2 the same from the M. luteus and M. guttatus reference samples, plus a typical flower from each of the range of Fi hybrids. Ali samples were flowered in the glasshouse together.

4 158 p. F. PARKER A comparison of the naturalized plants with the two parent species and their Fi hybrids shows that artificially raised hybrids have larger flowers and show a much greater variability in colouration. Conspicuous flower blotching is absent in the naturalized hybrids; the spotting on the lower lip in some is slightly more obvious than that found in M. luteus M. guttatus Fl Fig. I. Flower shape and patterning and leaf shape of M. guttatus, M. tuteus and their Fi hybrids. Flower size x.24, leaf size x.15. 2/7^ 1 n 12 2/7= Fig. 2. Flower shape and patterning and leaf shape of naturalized British Mimutus. i, Trawsfynnydd; 2, Codford; 3, S. Newton; 4, Coalville; 5, Yspyty Cynfyn; 6, New Radnor; 7, Monsal Dale; 8, Blackbrook; 9, River Lathkill; 1, Bowers Hall; 11, Galloway; 12, Harlech; 13, Llanengan, River Soch. the naturalized diploids, while in others there is one large central, dark red blotch on the lower lip. Flower size is variable, but in most of the naturalized hybrid samples, flowers are smaller than those of the experimental Fi and the throat is mainly closed as in M. guttatus. As far as the leaf shapes are concerned, the only obviously different naturalized

5 Mimulus in Britain hybrids are those from Harlech and Llanengan which have cuneate leaf bases. The leaves of M. luteus do tend to have a cuneate base, although this is not mentioned by Grant (1924) in her description of the five varieties then recognized. The experimental Fi hybrids all had leaves similar to those of the Harlech and New Radnor samples. The above observations on naturalized plants and experimental Fi hybrids show that although the chromosome number of the plants with reduced pollen fertility accords with their suggested origin as Fi hybrids between M. guttatus and M. luteus, their morphology is not strictly intermediate. With regard to flower type, the range in the naturalized plants examined is much less than in the Fi, with a bias towards the M. guttatus type. As the plants were collected on a random basis over a wide area, it would be expected that the sample, although small, would be reasonably representative. From a statement made by Roberts (1964), however, it appears that rather more variation should have been expected. If this is so then there are two possibilities: firstly, that the sample is unrepresentative, and secondly that there are local or regional differences with regard to the type of hybrid surviving. Mukherjee and Vickery (1962) have suggested that in the section Simiolus of the genus Mimulus there is one basic genome, n = 14, specifically of the type of M. guttatus. This is present in M. luteus in a slightly modified form, along with a second genome of unknown origin. As a wild species in South America M. luteus is little known, but it is apparently highly polymorphic; in Britain its distribution is limited to areas with cooler, damper summers (Perring and Walters, 1962). It may be predicted that the probability of a Fi hybrid successfully establishing and surviving in Britain will depend on how modified is the basic genome in its chromosome complement donated by M. luteus. Those Fi segregates with the highest proportion of modified genes will most resemble M. luteus in their climatic tolerances and fiower patterning, and these plants will be present as a higher proportion of surviving clones in the SW and NW of England; Wales and Scotland. In other parts of Britain, hybrid clones having a high proportion of unaltered genes in the basic genome and therefore more closely resembling M. guttatus, will have the higher probability of survival. The rate of clonal spread will depend upon both the climate and the habitat in which a hybrid successfully establishes itself. The open, continuous, gravelly habitats found along stretches of rivers in Derbyshire and elsewhere would allow a single vigorous clone to displace weaker hybrids. In this context it is probable that the samples from the River Lathkill, and Bowers Hall which is a short distance from this river, are parts of the same clone. In a habitat with denser, taller vegetation, and in areas more ecologically diverse, it is possible that several clones of varying fitnesses from one capsule of hybrid seeds could survive on different sites in the same area, at least for a time. It is highly probable that climatic variation and habitat interact to produce different variation patterns in Mimulus hybrids in different parts of Britain. Only a detailed cytotaxonomic survey of the genus in this country can provide the data to clarify the situation and thus test the validity of these hypotheses. REFERENCES GRANT, A. L. (1924). Monograph of the genus Mimulus. Ann. Miss. Bot. Gard., 11, 99. MCARTHUR, E. D., ALUM, H. T., ELDREDGE II, F. A., TAI, W. & VICKERY JR., R. K. (1972)- Chromosonie counts in section Simiolus of the genus Mimulus (Scrophulariaceae) IX. Polyploid and aneuploid patterns of evolution. Madrono, 121, 417. MCARTHUR E. D. (1974) The cytotaxonomy of naturalized British Mimulus. Watsonia, 15, i55maude, P. F. (1939). The Merton catalogue. A list of chromosome numerals of species ot British tlowermg 3) l NNew Phytol.,l % plditlxs. 2%, I.. o- 1 f tt,«npnnq MuKHERjEE, B. B. & VICKERY JR., R. K. (1959). Chromosome counts in the section Simiolus ot tne genus Mimulus (Scrophulariaceae) III. Madrono, 15, 57.

6 i6o P. F. PARKER MuKHERjEE, B. B. & ViCKERY, JR, R. K. (1962). Chromosome counts in the section Simiolus of the genus Mimulus (Scrophulariaceae) V. The chromosomal homologies of M. guttatus and its allied species and varities. Madrono, 16, 141. PERRING, F. H. & WALTERS, S. M. (1962). Atlas of the British Flora. Thos. Nelson and Sons. p ROBERTS, R. H. (1964). Mimulus hybrids in Britain. Watsonia, 6, 7. SALISBURY, E. J. (1964). Weeds and Aliens. Collins, London. TAI, W. & ViCKERY Jr., R. K. (1972). Unusual cytological patterns in microsporogenesis and pollen development of evolutionary significance in the Mimulus glabratus complex (Scrophulariaceae). Am. y. Bot., 59,488. ViCKERY, JR., R. K. (1955). Chromosome counts in the section Simiolus of the genus Mimulus (Scrophulariaceae). Madrono, 13, 17. Note added in proof McArthur (1974) gives counts of 2«= 45 for five samples of naturalized hybrids. Three from North Wales were similar to M. luteus, one from Yorkshire resembled M. guttatus. It is now apparent that all three cytotypes of M. luteus have been introduced.

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