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Hybridization and Introgression between Two Diploid Wild Relatives of White Clover, Trifolium nigrescens Viv. and T. occidentale Coombe

AgResearch Grasslands, Private Bag 11008, Palmerston North, New Zealand

View larger version (13K): [in this window] [in a new window]   Figure 1. Shikimate dehydrogenase zymogram patterns for (a) Trifolium nigrescens ssp. nigrescens, T. occidentale, F1 and backcross hybrids, (b) T. nigrescens ssp. petrisavii, T. occidentale, F1 and T. nigrescens ssp. nigrescens x (T. nigrescens ssp. petrisavii x T. occidentale) hybrids. In the T. nigrescens x (T. nigrescens x T. occidentale) progenies, both 1,1 and 1,2 phenotypes occurred.

View larger version (30K): [in this window] [in a new window]   Figure 2. Chromosome preparations of the F1 hybrid Trifolium nigrescens ssp. nigrescens x T. occidentale. (a) Giemsa-stained somatic metaphase cell (2n = 16). Eight larger chromosomes can be distinguished. Arrows indicate nucleolus organizer region (NOR)-bearing chromosomes. (b–f) Examples of fluorescence in situ hybridization (FISH) results. DNA probes are indicated with respective colors. Chromosomes are counterstained with DAPI. (b) A mitotic metaphase cell. Chromosomes with a secondary constriction (NOR) are indicated by arrows. (c) The same cell as in (b) after FISH with 5S and 18S rDNA sequences; FISH signals of the two probes on two NOR-bearing chromosomes are very prominent. A minute 5S rDNA signal on the short arm of a T. occidentale–derived chromosome is indicated by an arrowhead. (d) A mitotic prometaphase cell after FISH with TrR350. Five large centromeric FISH signals on T. occidentale-derived chromosomes are very evident. Some of the small signals are not clear. Dotted lines on two NOR-bearing chromosomes represent stretched secondary constrictions. (e) A meiotic diplotene cell shows eight bivalents and homeologous pairing of NOR-bearing chromosomes derived from the two parents. A T. occidentale–derived chromosome with a 5S signal (arrowhead) forms a bivalent, probably with a T. nigrescens-derived chromosome. (f) A meiotic pachytene cell with a single and prominent 18S rDNA signal denotes homeologous pairing of NOR-bearing chromosomes from the two parents. A minute 5S signal on a T. occidentale–derived chromosome is indicated by an arrowhead. Bar represents 10 µm.

View larger version (79K): [in this window] [in a new window]   Figure 3. Meiotic chromosome preparations of the F1 hybrid Trifolium nigrescens ssp. nigrescens x T. occidentale. (a) Metaphase I cell showing 8 bivalents. (b) Late anaphase I with 8–8 disjunction. Bar represents 10 µm.