1989. Cytogenetics and reproductive behavior of induced and natural tetraploid guayule (Parthenium argentatum Gray). Genome, 32: 1 100 -1 104. Tetraploid guayule (Parthenium argentatum Gray) plants with 72 chromosomes were obtained by colchicine application to the shoot apices of the diploid (2n = 36) seedlings. Interspecific hybridization of guayule as female with Parthenium rollinsianum Rzedowski (2n = 36) was used to compare the mode of reproduction of the induced tetraploids with that of natural tetraploids. Plants produced from crosses of induced tetraploid guayule and P. rollinsianum were all hybrids. Hybrid plants showed 2n = 54 chromosomes, indicating that the artificially produced tetraploids reproduce sexually. Interspecific crosses between natural tetraploids and P. rollinsianum produced predominantly maternal type progeny. This suggests that the apomictic mode of reproduction in guayule is a consequence of mutations which accumulated in the natural polyploids. Diploid guayule plants showed regular meiosis with an average of 14.86 rod and 3.14 ring bivalents. Induced tetraploids produced 0.25 I, 29.2 1 11, 0.13 111, and 3.24 IV. In comparison, natural tetraploids showed 1.20 I, 30.85 11, 0.06 111, and 2.23 IV. The frequency of multivalents, which was limited in the induced tetraploids, showed further reduction in natural polyploid guayule, which may indicate diploidization in these plants. Percent pollen stainability was 98.5 f 1.03, 83.7 f 12.24, and 89.5 f 3.4 1 for diploids, induced tetraploids, and natural tetraploids, respectively. Both induced and natural tetraploids had larger capitula, achenes, and pollen and thicker leaves than the diploids.Key words: guayule, natural rubber, reproductive behavior, meiosis, polyploid induction. 5HASHEMI, A., ESTILAI, A., et WAINES, J. G. 1989. Cytogenetics and reproductive behavior of induced and natural tetraploid guayule (Partheniurn argentatum Gray). Genome, 32 : 1 100 -1 104. Des plantes tktraploides de guayule (Parthenium argentatum Gray), ayant 72 chromosomes, ont Ct C obtenues par traitement h la colchicine des apex de tiges de plantules diploides (2n = 36). Une hybridation interspkcifique entre la guayule comme parent femelle et le Parthenium rollinsianum Rzedowski (2n = 36) a Ct C utilisCe pour comparer le mode de reproduction des guayules tCtraploides induits et celui des tCtraploides naturels. Les plantes rCsultant du croisement entre le guayule tetraploide induit et le P. rollinsianum se sont toutes av6rCes hybrides. Le nombre chromosomique de ces hybrides a Ct C de 2n = 54, ce qui indique que les guayules tCtraploides produits artificiellement se reproduisent sexuellement. Les croisements interspkcifiques entre les tCtraploides naturels et le P. rollinsianum ont produit une descendance qui Ctait de type maternelle de f a~o n prkdominante. Ceci suggkre que le mode apomictique de reproduction chez les guayule est une conskquence des mutations qui se sont accumulCes chez les polyploides naturels. La mCiose chez les plantes de guayule diploides a Ct C...

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Edible chia seeds, purchased from the local markets in Guatemala, Mexico, and southern California, were used for species identification, chromosome counts, karyotype construction, and meiotic analyses. Plants raised from those seeds had ovate leaves, dense racemose inflorescences, pale-blue flowers, and were identified from herbarium specimens as Salvia hispanica L. Mitotic analyses of root tips from 50 plants showed 2n = 12—the lowest chromosome number in the genus. Chromosomes were small, ranging from 2 to 3.5 pm. One pair of chromosomes was metacentric (with the long arm: short arm ratio, r = 1.5), four pairs were submetacentric (r = 2.6 to 3.6), and one pair was telocentric (r = 12). Meiosis was regular and six bivalents were observed at metaphase I. Ring and rod bivalents averaged 1.53 ± 1.05 and 4.47 ± 1.05, respectively;

F, hybrids obtained from crosses between the rubber-bearing species, Parthenium argentatum Gray (commonly known as guayule), and P. schottii Greenman ex Millspaugh and Chase, a nonrubber-producing species from southern Mexico, were morphologically variable. They were generally intermediate between the two species with respect to leaf size, head size, number of disk florets per head, and the length of the peduncles. Like the parental species, the hybrids had 2n = 36 chromosomes. They averaged 14.56 bivalents, 3.92 univalents, 0.56 trivalents, and 0.32 quadrivalents, indicating a high degree of homeology between the genomes of the two species. The observation of one quadrivalent at diakinesis in 32% ofthe PMCs and the frequent occurrence ofa bridge accompanied by a fragment at late anaphase I of the F, hybrids suggested that the two species differ in a reciprocal translocation and a paracentric inversion. The ease ofhybridization, the partial fertility ofthe hybrids, and their high degree ofchromosome pairing indicate that P. schottii and P. argentatum are closely related in spite of their distinct morphological features and geographical distributions. This close relationship provides an opportunity to transfer genes from P. schottii to guayule for desirable characteristics, such as high biomass production and resistance to Verticillium dahliae Kleb, by means of interspecific hybridization.