Dominant alleles of the locus Tl may differ in their strength
Bogdanova, V.S., Kosterin O.E. and Institute of Cytology and Genetics of Russian Academy of Sciences
Berdnikov, V.A. Acad. Lavrentiev ave. 10, Novosibirsk 630090, Russia
The Tendril-less (Tl) gene of Pisum sativum participates in controlling development of the compound leaf and is located in the linkage group V. A number of recessive alleles have been described. Homozygotes tlw, tlpet condition formation of leaflets in place of tendrils (2). Heterozygotes exhibit an intermediate structure that can be called 'flat tendril', that is tendril with a very narrow leaf blade. Another recessive allele tlx is supposed to be a small deletion (3), and only was observed in hemizygote which appears quite similar to the heterozygote Tl/tlw. However, little is known of variants of dominant alleles. Here we show that probably there exist at least two Tl alleles that differ in their strength, that is, visible expression in heterozygote against tlw.
In 1997 we performed a cross of the lines Delta and Sprint-Vologda as a part of investigation of the line Delta. The lines are described in (1), an important point for the present study is that Delta is heterozygous (r Tl/R tlx) and Sprint-Vologda is homozygous (R Tl/R Tl), the genes R and Tl being closely linked (4). For convenience, we will designate the Tl allele originating from the line Delta as TlD and that from the Sprint-Vologda line as TlSV. Thus, in the F1 from the cross Delta x Sprint-Vologda there appear plants with flat tendrils heterozygous for Tl (R tlx/R TlSV) and plants with round tendrils homozygous for Tl (r TlD /R TlSV). Plants of the latter class were test-crossed as a pollen parent with the line WL1018 (r tlw). We obtained 16 wrinkled (r tlw/r TlD) and 49 round (r tlw/R TlSV) seeds. The ratio of r to R seeds differed significantly from the expected 1:1, perhaps due to peculiarities of the line Delta (1). All plants resulting from this test-cross expressed flat tendrils. Visual comparison of flat tendril width in these two classes suggested a significant difference. We therefore measured length and width of flat tendrils in the leaves at node 8 in individual plants. The results are presented in Table 1.
The dimensions of the flat tendrils, particularly their width:length ratio, in two phenotypic classes, differ signi-ficantly. As one of the pheno-typic classes has a much higher variance for this character, it is reasonable to perform logarith-mic transformation. After this procedure, the difference between plants carrying TlD and TlSV alleles (more precisely, grown from r and R seeds) becomes more evident. Distri-butions of the logarithm of the
value (width/length x 100) for the tendrils of the ultimate pair are shown in Fig.1. The difference observed indicates that a factor controlling the width of flat tendrils co-segregates with the alleles of R locus. It could be some modifier closely linked to R and, therefore, to Tl. However, we believe the effect is more likely due to the Tl gene itself. The distributions overlap to some extent, probably due to high values of the variance or, possibly, misclassification of Tl-classes due to rare crossing-over between r and Tl (or between r and the hypothetic modifer). These results allow us to propose that the two wild-type Tl alleles studied in this work differ in their strength manifested in the width of flat tendrils of the heterozygote Tl/tlw. The TlD allele originating from the line Delta may be termed “weak”, and the TlSV allele that comes from the line Sprint-Vologda may be regarded as “strong”.
Acknowledgement: The work was partly supported by the Russian State Program ‘Russian Fund for Fundamental Research’, Grant No 99-04-49-970.
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