8. Viviparous.
The studies of the viviparous mutants of corn have
yielded some information as to the loci of the genes involved.
Indications are that the mutants vp1 and vp4 obtained from Cornell are allelic. Linkage studies
give no indications of this gene being located on the short arm of chromosome
nine or the long arm of chromosorme ten. These are the loci for vp4 and vp1 respectively, reported in the literature. Crosses
of stocks of this mutant with translocations B‑9a, B‑9b and B‑10
have revealed that the gene is not within the regions included in these
translocations. Crosses with C, R, wx and sh give no
indication of linkage with any of these genes. Apparent linkage with aleurone
color, previously reported for this mutant, seems to be the result of the
inhibition of color development when the gene is present in the homozygous
condition.
A backcross of vp2 has yielded 29.5% crossing over between vp2 and pr,
which is in good agreement with values previously reported.
Previous crosses of vp5 with translocation B‑1b had indicated that
this mutant was in the short arm of chromosome one. A backcross of an F1
heterozygous for vp5
and translocation 1‑9a indicate 29.9% croosing over between the mutant
and the translocation.
A three point test of vp7, pr and gl8 indicates the follawing order and map distances: vp7 14.6 pr
1.0 gl8. This data is
in agreement with the order of these genes indicated by Dr. Sprague.
Viviparous had previously been located in the long
arm of chromosome one with the aid of translocation B‑1a. Backcrosses of
F1 plants heterozygous for this mutant: br, bm2 and translocation 1‑7c indicate the following
order and map distances: (br) T
32.5 vp8 13.2 bm2. No crossovers were obtained between br and the translocation out of of 103 plants observed.
F2 data from plants heterozygous for vp9 and translocation 7‑9a‑76 indicate that
this mutant is on chromosome seven. Crosses with translocation B-7b give no
evidence of the gene being included in the translocated portion of chromosome
seven, thus the gene is probably proximal to the break in this chromosome.
Physiological studies of normal and viviparous
embryos seem to indicate that the combimtion of low oxygen tension and high CO2
tension is responsible for preventing germination of normal embryos during the
development of the caryopsis. Concentrations of approximately 1% O2,
30% CO2 and 69% N2 will greatly retard the elongation of
the plumule of excised normal embryos of immature seed without inhibiting the
enlargement of the scutellum which will elongate from .5 to 1.5 mm in six days
under these conditions. The plumules of vp5 embryos, obtained from the same selfed ear as the
normals mentioned above, are not inhibited by these gas concentrations. This
would suggest that embryos of this mutant germinate prematurely because they
are capable of plumule elongation under conditions of low and high CO2,
which possibly exist within the pericarp of developing seeds.
D. S. Robertson