in the Pyrenees

THE DISTRIBUTION OF DIPLOID AND TETRAPLOID CYTOTYPES OF PLANTAGO MEDIA IN THE PYRENEES1

Peter VAN DIJK2

RESUMEN.—Las razas diploides y tetraploides de Plantago media muestran en el Pirineo una distribución parapátrica, la cual se relaciona sin duda con los factores ambientales. En esta nota se discute, como explicación más verosímil, la existencia de diferentes modelos de colonización —a fines del Pleistoceno— seguidos de exclusión por interacciones ecológicas y genéticas. RÉSUMÉ.—Les races diploTdes et tetraploides de Plantago media nous montrent dans les Pyrénées une distribution parapatrique que ron peut sans doute mettre en rapport avec les facteurs de l'environnement. Nous proposons ici comme une explication trés vraisemblable, l'existence —vers la fin du Pleistocéne— de différents modéles de colonisation suivis d'exclusion par interactions écologiques et génétiques. SUMMARY.—Diploid and tetraploid Plantago media have a parapatric distribution in the Pyrenees. This parapatry is unlikely to be caused by environmental factors. It is discussed that different colonization patterns at the end of the Pleistocene followed by exclusion due to ecological and genetic interactions is a more likely explanation.

INTRODUCTION

Polyploidy has played an important role in plant evolution. Many species exhibit intraspecific polyploidy and on the basis of chromosome numbers, it has been estimated that 47% of the Angiosperms are of polyploid origin (GRANT, 1981). Plantago media comprises two cytotypes: diploids (2n=2x=12) and tetraploids (2n=4x=24). Both cytotypes can cross-fertilize, but 3x hybrid seeds are aborted. Severa! lines of evidence indicate that 4x Plantago media is of autotetraploid origin (VAN DIJK &

VAN DELDEN, in press): 1. strong morphological resemblance

1 Grassland Species Research Group Publication 169. 2 Institut voor Oecologisch Onderzoek. Koninklijke Nederlandse Akademie van Wetenschappen. Boterhoeksestraat 22. 6666GA, postbus 40. NL-6666 ZG Heteren.

2. break down of the post zygotic crossing barrier by colchicine treatment 3. production of rare 3x and 4x hybrids between 2x and 4x 4. formation of trivalents in 3x hybrids 5. identical allozyme and CpDNA polymorphisms. Today the 4x cytotype, that arose from the 2x cytotype, occupies most of Western and Central Europe (Figure 1). RAHN (1954) reported on 2x and 4x populations in the Pyrenees. The goal of this study was to describe the distribution of the cytotypes in the Pyrenees in detail, as part of a larger study on the evolutionary significance of autopolyploidy in Plantago media.

MATERIAL AND METHODS

Seeds were collected randomly from over 46 populations throughout the Central Pyrenees in 1986 and 1988. The ploidy level was determined in seedlings of 5-20 seed parents per population (root tip mitosis).

RESULTS AND DISCUSSION

Only 2x and 4x plants were found in the present survey. The distributions are shown in Figure 2. The distribution of 2x and 4x in the Pyrenees is essentially parapatric. Diploids occupy the Western Pyrenees, tetraploids occupy the Eastern Pyrenees. The 2x range links up with the occurrence of 2x around Bilbao (RAHN, 1954) and in the Picos de Europa (own observations). Diploids are also found in the low land areas, north of Pau. The 4x range in the Pyrenees seems to link up with the 4x distribution in Central France. However, the distributions of the cytotypes outside the Pyrenees in Spain and France are largely unknown at the moment. It would be of interest to know whether or not the parapatric distributions continue in the lowland areas.

Both cytotypes grow in neutral to alkaline grasslands between sea level and ±2.000 m altitude. The preference for alkaline soils explains the almost complete absence of P. media in the French Hautes Pyrenees. It is unlikely that the distribution of the cytotypes is caused by differences in ecology between the cytotypes, given the variation in habitats within the ranges of both cytotypes. In a narrow zone of overlap between the cytotype ranges, severa! sympatric populations have been found. For instance, south of El Formigal 2x and 4x are sympatric over a distance of at least 5 km.

It is suggested that the present-day distribution of the cytotypes is strongly affected by historical factors. Detailed investigations of distributions of allozymes in the snail Cepaea nemoralis and of subspecies in the grasshopper Chorthippus parallelus reveal clear patterns of colonization of the Pyrenean range at the end of the Pleistocene (OcHmAN & al., 1983; HEWITT, 1989). Presumably, after the Pleistocene 2x Plantago media colonised from the west, where as 4x colonised from the east. This may have taken place between 10.000 and 8.000 BP (MONTSERRAT & VILAPLANA, 1987) and therefore the parapatric distribution cannot be explained by lack of time to mix up. Additionally human activities in the past, like deforestation (JALUT, 1988), will have had their effects on the distribution, but probably on a more local scale. Consequently the parapatric distribution of cytotypes is best explained by mutual exclusion. This could be achieved by the occupation of the niche by the other cytotype (ecological exclusion), or by exclusion of the minority cytotype sensu LEVIN (1975) due to frequency dependent production of non-viable 3x seeds (genetic exclusion). In an out

The distribution of 2x and 4x cytotypes of Plantago media in Europe, based on own chromosome number counts and data from literature ; sympatric 2x-4x populationsel). A ; 4x populations ■ (from VAN DIJK & VAN DELDEN, in press) (2x populations

Figure 1.

FRANCE

50 km

SPAIN

; sympatric 2x-4x ■ 4x populations

A ;

in the Pyrenees (2x populations Plantago media Figure 2. The distribution of 2x and 4x cytotypes of populations ).

crossing plant species, plant species, like Plantago media, genetic exclusion could be an important factor.

ACKNOWLEDGEMENT

This research was supported by the Foundation of Fundamental Biological Research (BION), which is subsidized by the Netherlands Organization for Scientific Research (NWO).

BIBLIOGRAPHY

GRANT, V. (1981). Plant speciation. Columbia University Press. New York. HEWITT, G.M. (1989). The subdivision of species by hybrid zones. In: OTTE, D. & ENDLER, J.A. (eds). Speciation and its consequences. Sinauer Associates. Sunderland. Massachusetts. pp 85-110. JALUT, G. (1988). Les principales étapes de l'histoire de la forét pyrénéenne frangaise depuis 15.000 ans. Monografías del Instituto Pirenaico de Ecología, 4: 609-615. LEVIN, D.A. (1975). Minority cytotype exclusion in local plant populations. Taxon, 24: 3543.

MONTSERRAT, J. & VILAPLANA, J.M. (1987). The paleoclimatic records of the upper pleistocene and holocene in the Llauset valley (central southern Pyrenees). Pirineos, 129: 107-113.

OCHMAN, H.; JONES, J.S. & SELANDER, R.K. (1983). Molecular area effects in Cepaea. Proc. NatL Acad. Sci. USA, 80: 4.189-4.193. RAHN, K. (1954). Experimental and cytological studies in Plantago media. Bot. Tidsskr, 51: 300-307. VAN DIJK, P. & HARTOG, M. (1988). The geographical distribution of cytotypes of Plantago media in the Pyrenees and its importance for the study of the evolutionary significance of polyploidization in plants. Documents d'Écologie Pyrénéenne V: 121131.

VAN DIJK, P. & VAN DELDEN, W. (1991). Evidence for autotetraploidy in Plantago media and comparisons between natural and artificial cytotypes concerning cell size and fertility. Heredity (in press).