Cytological and cytogenetic studies were carried out on populations of leafy spurge (Euphorbia esulaL. s.l.) and its allies primarily from North America but also from Europe. Chromosome numbers were determined for 126 samples ofE.esula, 11 ofE.cyparissiasL., 1 of the hybrid (E. times;pseudoesulaSchur) between these two species, and 1 ofE.agrariaBieb. All plants ofE.esulawere hexaploid. Of the total, 125 leafy spurge accessions had a chromosome number ofnensp;=ensp;30; 1 ofnensp;=ensp;25++. Very few meiotic abnormalities were observed.Euphorbia cyparissiaswas primarily tetraploid (nensp;=ensp;20), although occasional diploids (nensp;=ensp;10) were encountered. Tetraploids were fertile; diploids were sterile. The hybrid between the two foregoing species had a chromosome number ofnensp;=ensp;25, indicating that theE.cyparissiasparent was a tetraploid; meiosis in the hybrid was abnormal.Euphorbia agrariawas found to have a gametic number ofnensp;=ensp;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 F1progeny 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 thatE.esulais 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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