Copyright 1998-2017 Yoshiaki Yoneda
It was reported that the number of somatic cell chromosomes of Ipomoea nil was 2n=30 in 1928 and that there were 15 bivalent chromosomes in reduction division.
However, the morning glory chromosomes are so small that their numerical determination was possible only with observation by a paraffin sectioning method. Gene analysis of the morning glory progressed from about 1919, and a study of its chromosomes started shortly thereafter. A study of the forms of somatic cell chromosomes is also extremely difficult because of the chromosomes' small size. G. Nakajima observed chromosomes of a root tip in 1963 by this method: A root tip was pretreated by 8-oxyquinoline, then hydrolyzed in mixed liquid of acetic orcein and hydrochloric acid, and the hydrolyzed tissue was smeared on a slide glass. The results were reported as follows: among 20 strains, the common strain called the prototype had 15 pairs of chromosomes, ranging from 3.50 ~ 1.45 micro-meters in length, among which 4 pairs had satellites (SAT chromosome), one pair had median constriction (kinetochore is located centrally in a chromosome) and 10 pairs had terminal constrictions. In the rest of the gardening strains, the number of SAT chromosomes and the morphology of chromosomes collectively (karyotype) were slightly different from strain to strain. However, this method of staining the whole chromosome uniformly can distinguish each chromosome only with a position of constriction (kinetochore) of a chromosome and the presence of a satellite. It is therefore difficult to catch relationships between strains.
In the 1970s, a method to stain a chromosome differentially (stain it into bands of light and shade) was developed. The chromosomes of Ipomoea nil were stained with a new method developed recently. The following method is used: After pretreatment with 8-oxyquinoline solution, a root tip was macerated with enzyme mixture liquid of cellulase and pectolyase, and the macerated tissue was transferred to a slide glass with the addition of acetic acid-methanol and then flamed to spread each cell. The cells were then stained with Giemsa solution. One example of prometaphase in a root tip cell of the Tokyo Kokei Standard strain is shown in the figure. Among 15 pairs of chromosomes, ranging from 1.8 ~ 5.1 micro-meters in length, 6 pairs had median constriction, 7 pairs submedian, and 2 pairs subterminal. There was a satellite in the twelfth (counting from the largest) chromosome. In addition, the end of the third chromosome became condensed frequently, and looked sometimes like a satellite body.
(Kawamura / Yoneda, original figure)
|Chromosomes and karyotype
(Kawamura / Yoneta, original figure)
Gene Linkage Map
Because there are 30 somatic cell chromosomes of Ipomoea nil, 15 linkage groups would be expected. So far only 10 groups have become clear. Since 1916, over 220 genes have been analyzed, among which more than 140 genes have been divided into 10 linkage groups, and the loci of more than 70 genes were settled.
|Gene linkage map|