Leafy spurge and cypress spurge are important weeds of pastures and unimproved rangeland in western and eastern North America, respectively. Taxonomists disagree as to whether leafy spurge is a single variable species or an aggregate of two or more species. Morphometric techniques (clustering by incremental sum of squares and principal coordinate analysis) were used to analyze relationships in leafy spurge and its allies. On the basis of studying 26 morphological characters found in 200 collections representing 32 putative taxa, we concluded that in North America only four species should be recognized, namely, Euphorbia agraria Bieb., Euphorbia cyparissias L., Euphorbia esula L., and Euphorbia ×pseudoesula Schur. A key to these taxa is provided. No authentic material of Euphorbia lucida Waldst. & Kitt. and Euphorbia salicifolia Host was found from this continent in the collections we examined.
Chromosome number determinations are reported for 58 accessions, comprising 13 native and introduced taxa of Artemisia found in Canada and the United States. Chromosome numbers observed were n = 8, 9, 18, and 27. A chromosome number of 2n = 18 is the first report for A. pacifica Nutt. A new tetraploid cytotype (2n = 36) was found in A. frigida Willd. Supernumerary chromosomes (n = 9 + 3) and mixoploidy (n = 18, 36) were also observed in this taxon for the first time. Heteroploidy was present in several species. In some taxa, morphological or ecological differences between the chromosomal races were detected; in other cases no differences were noted. Karylogical and phylogenetic evidence is presented for the original chromosome number in Artemisia having been x = 9.
Cytological and cytogenetic studies were carried out on populations of leafy spurge (Euphorbia esula L. s.l.) and its allies primarily from North America but also from Europe. Chromosome numbers were determined for 126 samples of E. esula, 11 of E. cyparissias L., 1 of the hybrid (E. ×pseudoesula Schur) between these two species, and 1 of E. agraria Bieb. All plants of E. esula were hexaploid. Of the total, 125 leafy spurge accessions had a chromosome number of n = 30; 1 of n = 25++. Very few meiotic abnormalities were observed. Euphorbia cyparissias was primarily tetraploid (n = 20), although occasional diploids (n = 10) were encountered. Tetraploids were fertile; diploids were sterile. The hybrid between the two foregoing species had a chromosome number of n = 25, indicating that the E. cyparissias parent was a tetraploid; meiosis in the hybrid was abnormal. Euphorbia agraria was found to have a gametic number of n = 20, which is the first chromosome number determination for this species; meiosis was normal. Artificial crosses were made successfully between 31 accessions of leafy spurge. Seed germination of the F1 progeny slightly exceeded that reported for natural populations, and meiosis was normal. Pollen stability studies were carried out on herbarium material. Stainability was 100% for most of the samples studied. Extensive pollen size polymorphism was found. It is suggested that this phenomenon supports the hypothesis that E. esula is of allopolyploid origin. No cytological or cytogenetic basis was found for considering the leafy spurge accessions examined in this study as other than as a single, albeit somewhat polymorphic, species.
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