Euploid and aneuploid evolution in Potamogeton (Potamogetonaceae): a factual basis for interpretation
Introduction
In a recent account of Potamogeton in the British Isles by Preston (1995), Hollingsworth (1995)referred to some unpublished chromosome counts made on native material of seven taxa, of which details have since been published for P. coloratus Hornem. (2n=ca. 26: Hollingsworth et al., 1995a) and P. natans L. (2n=ca. 52: Hollingsworth et al., 1995b). One aim of this paper is to present details of the counts for the other five taxa, which comprise three species (P. pectinatus L., P. perfoliatus L., P. polygonifolius Pourr.) and two hybrids (P.×nitens Weber and P.×salicifolius Wolfg.). A second aim is to interpret the counts against the cytological background of the genus as a whole, commenting on evidence regarding the original base number for the genus and on the frequency of aneuploidy. To this end we present a review of the chromosome numbers that have been reported for species of Potamogeton.
Some explanation is needed as to why a cytological review of Potamogeton is necessary when a similar exercise was conducted not long ago by Les (1983), who attempted to assess the taxonomic value of such data and to identify evolutionary trends within the genus. His work was criticised, however, by Wiegleb (1988)on the grounds that the listing of chromosome counts was incomplete, the attribution of synonymy was incorrect in some cases and the assignment of species to morphological groups was erroneous. As far as chromosome numbers are concerned, Les (1983)relied on the various indices and compilations for his information and apparently did not routinely consult the original publications. While these abstracts provide a general picture and Les' purpose was only to find “the most commonly reported numbers” for each species (Les and Sheridan, 1990), use of the secondary literature presents many pitfalls. Misleading summaries supplied in original publications can engender errors if used uncritically when abstracted; errors of transcription become incorporated into the indices and compilations; some secondary sources adopt a synonymy that is either unacknowledged or erroneous, which can lead to confusion; and none of the secondary sources listed gives details of the number of populations investigated by the original study. Simply adding up the number of reports for each species based on secondary sources, therefore, may for a variety of reasons result in a mistaken impression of the frequencies of particular numbers. Finally, it is always assumed that plant identifications have been correctly made, which may not be the case, especially with Potamogeton, and is clearly a hidden source of potential error. With one exception, the present review draws its data exclusively from primary sources in order to avoid all but the last of these problems and presents for the first time a detailed review of what is known (or at least what is claimed to be known) about chromosome numbers in the genus Potamogeton.
Section snippets
Chromosome counts
Vegetative material of P. pectinatus L., P. perfoliatus L., P. polygonifolius Pourr. and two hybrids, P.×nitens Weber (P. gramineus×perfoliatus) and P.×salicifolius Wolfg. (P. lucens×perfoliatus) was collected from the localities listed in Table 1. Root tips from one individual per population were pretreated in 0.2 mM 8-hydroxyquinoline at 4°C for 24 h, and then fixed in a mixture of absolute ethanol and glacial acetic acid (3:1 vol:vol). Squashes were made after the root tips had been
Chromosome counts
Our chromosome counts are summarised in Table 1. Chromosome counting in Potamogeton is difficult and the small, dot-like chromosomes, frequently only one micron long, make the potential for error high. It is particularly hard to distinguish genuine agmatoploid variation from that caused by technical difficulties and we therefore regard many of our results as approximate, e.g., as in the case of P. pectinatus, where 2n=ca. 78. The exceptions are counts of P. polygonifolius, for which clear
Chromosome counts
Our counts for P. pectinatus (2n=ca. 78) and P. perfoliatus (2n=ca. 52) are in agreement with those in the literature (Table 2, Table 3). Our findings of 2n=ca. 52 for P.×nitens (P. gramineus×P. perfoliatus) and P.×salicifolius (P. lucens×P. perfoliatus) are also consistent with what would be expected based on the chromosome numbers of the parental species. Our only chromosome counts which disagree with other published records are, paradoxically, the ones about which we can be exact, namely
Acknowledgements
We are grateful to J.P. Bailey and K.I.A. Al-Shammary for assistance with the cytological techniques; to Ruth Dempsey for helping to locate some references; to L. Shlumukov for translating articles in Russian; and to G. Wiegleb for an unpublished summary of the nomenclature of Potamogeton.
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