The Genus Collinsia. XIII. Cytogenetic Studies of Interspecific Hybrids Involving Species with Pediceled Flowers

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Abstract
1. Thirty interspecific hybridizations involving twelve species of Collinsia Nutt. (Scrophulariaceae) were attempted; twelve crosses gave seed that germinated. 2. Seed from the cross between C. sparsiflora subsp. arvensis and C. multicolor gave seedlings that died at the cotyledon stage or soon after the first pair of leaves was evident; seed from C. solitaria x C. greenei germinated in the immature capsule and had to be planted prior to the maturation of the capsule. 3. Except for the hybrid between C. sparsiflora subsp. arvensis and C. bruceae which had bivalents and, occasionally, univalents, the interspecific hybrids had one or two interchange complexes with four, six, eight, or ten chromosomes at metaphase I; except for the hybrids within the group containing C. sparsiflora, C. bruceae, and C. solitaria, the interspecific hybrids had two or three dicentric chromatid bridges and fragments at anaphase I, indicating the presence of heterozygous paracentric inversions. 4. The two interchange complexes in hybrids between C. solitaria and C. sparsiflora subsp. arvensis or C. bruceae displayed a directed orientation at metaphase I; the interchange complex or complexes in the other interspecific hybrids did not. 5. The progeny of the hybrids between C. solitaria and C. sparsiflora subsp. arvensis or C. bruceae included plants with bivalents, with one complex, or with two complexes in the expected ratio of 1:2:1; the complexes displayed a directed orientation as in the hybrids. 6. The mean number of chiasmata in hybrids within the group containing C. sparsiflora, C. bruceae, and C. solitaria was relatively high and similar to the values for the species; the mean number of chiasmata in the other interspecific hybrids was relatively low, regardless of the chiasmata frequency of the species involved in each hybrid. 7. Hybrids within the group containing C. sparsiflora, C. bruceae, and C. solitaria were fertile; the other interspecific hybrids were sterile. 8. Many interspecific hybrids displayed heterosis and often combined morphological characters found in the parental species; hybrids involving species in the group containing C. sparsiflora, C. bruceae, and C. solitaria did not differ markedly from the parental species. 9. Spontaneous amphiploid branches were detected in an interspecific hybrid and in the progeny of an interspecific hybrid; two spontaneous pentaploids were observed in the progeny of a self-pollinated plant of C. bruceae. 10. Supernumerary chromosomes were found in the hybrid between C. sparsiflora subsp. arvensis and C. bruceae. 11. A diploid chromosome number (n = 7) and a relatively low chiasmata frequency are reported for C. linearis, a species with pediceled flowers. 12. Chromosome formulas for eight species are presented to account for the number of interchange complexes and the number of chromosomes in each complex in the interspecific hybrids. 13. The role of chromosomal repatterning in speciation in Collinsia is discussed.

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