328 Cytologia 32

A Chromosome Atlas of the Collective Genus

Askell Love and B. M. Kapoor

Department of Biology, University of Colorado, Boulder, Colorado, U. S. A.

Received May 12, 1967

The genus Rumex, in the collective sense of most manuals, contains almost seven hundred described representing only about one hundred seventy accepted biological species, among which are some of the most widespread of the world. Its species are as frequent in dry and sandy soil as they are in marshes and cultivated fields, and they occur throughout the tropical, subtropical, temperate, boreal, and subarctic regions, and a few species even grow in the arctic zone. Some taxa are strictly maritime or insular, while others are exclusively alpine or continental; many evince no particular edaphic or climatic preferences, whereas others are extremely specific in their ecological requirements. There are species reaching seven meters in height and others rarely surpassing a few centimeters, and the variation in morphological characters is so great that the reason for the allocation of all these taxa to only a single genus sometimes is difficult to comprehend. Although several characteristics of the vegetative parts of the taxa of the genus Rumex have been used as a basis for classification, the amazing variability of the inner perianth segments, or valves, of the fruit has been found to be of an utmost importance for the grouping of the genus into taxa at all lower levels. These segments, which are small and inconspicuos in the flower, increase and change their shape in various ways during the development of the fruit. Since these changes are clearly connected with some characters of the vegetative parts of the plants, there seem valid reasons to believe that they are of an evolutionary significance and, thus, may form a valuable basis for the natural classification of the species of this variable genus. There are indications, however, that some parallel evolution of these characters may have taken place, thus diminishing their importance for sectional classification, but more experimental evidence is needed before such a supposition can be used as a basis for a drastic revision of the sectional classification

presently accepted. Though we know next to nothing about the evolutionary sequence of various combinations of valve characteristics, it is generally alleged that the most primi tive type of valves is that which is membranous with a network of nerves, whereas those with a thickened midrib transformed into a callosity on one or all three valves are supposed to be derived. Valves with teeth of various sizes and forms are regarded as most advanced, especially when combined with the callosity of the 1967 A Chromosome Atlas of the Collective Genus Rumex 329 second type. It may be a support for this supposition that toothed valves with callosities characterize annual species which are regarded as more advanced than the perennial taxa, although this argument may be weakened by the occurrence of teethless valves in some annuals , which are then regarded as reduced types. There is ample morphological evidence in support of the suggestion that the annual condition is derived from the perennial one . Most classifications of Rumex into sections are based on such allegations , which still remain without paleobotanical support. There is, however, hardly any valid reason to doubt that the grouping of the genus Rumex in its strict sense proposed by Rechinger (1937, 1949a, b, 1954) on basis of the supposition that the more the inner perianth segments are transformed during the ripening process the more advanced is the species, are considerably more natural than the arrangement , by Meisner (1856), of his Lapathum section into two groups, based on their having entire or dentate valves. Need for some caution in the use of these characters, however, is indicated by observations made by Sarkar (1958) on the Axillares group. The collective genus Rumex has been divided into two main groups by laymen since times immemorial, i.e. into the mainly dioecious or monoecious and the hermaphroditic docks. This division was accepted, at the sectional level, by Campdera (1819), the first monographer of the genus. Later, Meisner (1856), the second and only other monographer of the group, divided it into the three sections, Acetosa, Acetosella, and Lapathum, a division accepted by later authors, until Rechinger (1937) raised the sections to the level of three subgenera. All these classifications are firmly based on morphological distinctions. The additional cytological evidence available to Love (in Love and Love 1948) made him conclude that it would be more appropriate to accept a generic status for Acetosa and Acetosella, as proposed by Miller (1759) and Fourreau (1869), at the same time as he proposed that the sections Platypodium of Willkomm (in Willkomm and Lange 1861) and Axillares of Rechinger (1937) be given subgeneric status, equivalent to Lapathum, within the restricted genus Rumex. This went unobserved by Rechinger (1949a), who accepted the groups as sections only, and also by Rechinger (1954), when he proposed a subgeneric status for PlatyPodium and maintained Acetosa, Acetosella, and Lapathum as comparable subgenera. The most recent step in this gross classification of the main groups of the collective genus was taken by Love and Love (1961), who distinguished between the four equivalent genera Acetosa Mill., Acetosella Fourr., Rumex L., and Bucephalophora Pau, the last one being the former section or subgenus PlatyPodium. It is the opinion of the present writers that this is in conformity not only with the clear morphological and cytological distinctions of these groups (cf. Lousley 1949), but also with the fact that their genetical differences are so profound that they prevent all hybridization between them. It is possible and indeed likely that these four genera may have had a common origin, though they have differentiated so far that their evolutionary relationships cannot be retraced by aid of any 330 A. Love and B. M. kapoor Cytologia 32

Table 1. 1967 A Chromosome Atlas of the Collective Genus Rumex 331

Table 1. (cont.) 332 A. Love and B. M. Kapoor Cytologia 32

Table 1. (cont.) 1967 A Chromosome Atlas of the Collective Genus Rumex 333

Table 1. (cont.)

Cytologia 32, 1967 23 334 A. Love and B. M. Kapoor Cytologia 32

Table 1. (cont.) 1067 A Chromosome Atlas of the Collective Genus Rumex 335

Table 1. (cont.)

23 * 336 A. Love and B. M. Kapoor Cytologia 32

Table, 1. (cont.)

available method. Some nomenclatural changes required by the acceptance of the genera Acetosa and Bucephalophoyaare being proposed elsewhere by Love and Evenson (1967) and Love and Kapoor (1967). The natural genera split out of the collective genus Rumex vary in size, from Bucephalophora with its single, variable species, and Acetosellawith a few taxa, to Acetosa and Rumex s. str. to which the bulk of described species belong. The 1967 A Chromsoome Atlas of the Collective Genus Rumex 337

species of each of these latter two genera can be grouped into some units at various levels, each including species which may be hybridized and , thus, certainly are closely related. Though Love (in Love and Love 1948) proposed the subgeneric level for the main groups of Rumex s. str. and Acetosa, it seems more appropriate to classify them as sections with subsections, as proposed by Rechinger (1937, 1949a, b, 1954) and Love (1944), since at least in Rumex s. str. crossability barriers have not yet become absolute between even the major groups . The genus Rumex in the wide sense has been of interest to biosystematists for several reasons. It was among the first groups to be studied cytologically with an evolutionary classification in mind (Roth 1906), although the chromosome numbers first reported were inexact ; few other genera have been studied more intensely from these points of view, though several of its taxa still remain cytologically unknown. It was in connection with studies on Rumex that Kihara and Ono (1926) proposed the distinction between auto and allopolyploidy. When distinct sex chromosomes were first discovered in vascular plants, Kihara and Ono (1923b, 1925) reported these for Acetosa pratensis, and later studies on sex determination mechanisms have leaned heavily upon various species of Acetosa and Acetosella (Jackson 1967; Jensen 1936b; Kihara and Yamamoto 1932; Love 1940a, b, 1941b, 1943a, b, 1944, 1957; Love and Sarkar 1956; Meurman 1925a, b; Ono 1926a, 1930b, 1932, 1935; Ono and Shimotomai 1928; Pazourkova 1966; Shimizu 1961; Smith 1955, 1957, 1958, 1963, 1964a, b, 1967; Smith and Smith 1947; Takenaka 1937, 1956; Yamamoto 1933a, 1935a, b, c, 1938; and Zuk 1963). The present account is designed as a review, in tabular form, of chromosome number determinations from taxa of the collective genus Rumex, excluding reports which later have been found to be inexact but listing also species and subspecies which still are cytologically unknown, in the hope that some of our readers may be able to furnish us with material to fill in these gaps. This paper is a step in intensified studies of the general processes of evolution of the species of all the groups of the collective genus, and of the particular processes of diversification of sex mechanisms in its dioecious and monoecious taxa. No other comparable genus of higher plants seems to be better suited for such a concerted attack on the phenomena of gradual and abrupt speciation, tribal, generic, and sectional differentiation, and the evolution of all kinds of cytogenetical sex determination mechanisms, because of its size and extreme variability. This work has been supported by NSF grant GB-3774.

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