TENDRILLED ACACIA (tac): AN ALLELE AT THE Uni LOCUS

PNL Volume 18 1986 RESEARCH REPORTS

Marx, G. A. NYS Agricultural Experiment Station, Geneva, NY USA

Tendrilled acacia (tac) recently has been found to be situated in

chromosome 3 (5), toward the M end of the chromosome (6). To fix its

position more precisely, tac was crossed with lines which were known or

thought to reside in the segment between st and uni. It became apparent

from these crosses that tac and uni are allelic.

Analysis of crosses involving st apu tac and M (Table I) revealed a

rather close linkage between M and tac, and the close association between

st and apu, reported previously (5), was confirmed. Since earlier evidence

(3,4) indicated that we 1 is located close to st, in the region between st

and M, we1 was also used in crosses with tac. The present evidence,

however, indicates that wel lies toward the b end of chromosome 3 rather

than toward the M end. The substantial population size in the present

studies and the fact that the cross is a four point cross lends con-

siderable credence to the new findings.

Tables 3-4 provide additional linkage data for chromosome 3 markers

collected in the course of pursuing studies not directly related to this

investigation.

Since M and uni are known to be closely linked (2) and inasmuch as tac

and M are closely linked (Table 1), tac and uni were expected to be closely

linked as well. To test this assumption, tac was crossed with WL 187 (type

line for uni), seeds of which were kindly supplied by Dr. Blixt. Since

uni/uni plants are sterile, it was necessary to use a number of phenotypi-

cally normal segregants in WL 187 as parents to ensure recovering the uni

allele from a heterozygous plant. A total of 73 F1 seeds from 11

individual F1 plants were planted. Six of the F1 progenies contained wild-

type plants exclusively whereas five progenies contained plants of two

different phenotypes: some wild type and some resembling a combination of

tac and uni. In plants of the latter class the early-formed leaves were

unifoliate; later leaves became tripartite, followed by two pairs of

leaflets and a terminal leaflet. These plants characteristically had no

terminal tendrils, but rarely an odd subterminal tendril did appear.

Flowers on the affected plants were somewhat malformed In contrast to the

flowers borne on sibling plants. The malformation notwithstanding, the

flowers more closely resembled normal flowers than the distinctively

sterile inflorescences of uni/uni plants. Fertility was only somewhat

impaired. The described plants were interpreted as carrying the tac allele

and the uni allele together in the same plant, thus evidencing alleles at a

single locus.

To pursue this supposition further, and to demonstrate that the plants

in question were not selfs, I grew the selfed seed of two of the F1's

showing the described hybrid phenotype. One population contained 44 F2

plants (one died early), the second 18 plants (Table 5). The spectrum of

plant phenotypes was the same in both populations. There were no normal,

wild-type plants in either population. Most plants resembled tac (some

with typical subterminal tendrils) and a minority resembled the distinctive

phenotype conferred by uni/uni, including the typical malformed, sterile

PNL Volume 18 1986 RESEARCH REPORTS

inflorescences (Table 6). The tac allele apparently is dominant to the uni

allele but the dominance is incomplete and the heterozygotes usually can be

distinguished from tac homozygotes (and readily from uni homozygotes).

Segregation for these phenotypes was accompanied by normal segregation for

other markers present in the cross (Table 6); segregation also occurred at

the R and I loci.

Even before recognizing the stated allelism certain similarities were

evident between tac and uni plants in the early seedling stage. The first

true leaf of tac plants often is unlfoliate and, conversely, uni typically

bears some tripartite leaves, although later in development.

Another feature of tac plants is the reduced number of leaflet pairs.

In fact, this is a convenient means to distinguish tac tl from Tac tl in

segregating populations. Sharma (7) has already pointed out that the

terminal leaflet of tac plants is not appreciably reduced in size as is

typically the case in _t_l plants. In plants homozygous for tac and for tl,

the pair of subterminal tendrils (characteristic of tac) are absent, thus

for this property tl is epistatic to tac. However, tac Tl plants often

show variability of expression with respect to the subterminal tendrils;

not infrequently the leaves have only one tendril or they may have none.

Thus the absence of subterminal tendrils is by itself not a sure way to

distinguish between tac t1 and Tac t1 plants. Such a distinction can

confidently be made, however, by observing differences in the number of

leaflet pairs per leaf. Plants with the tac t1 combination have fewer

leaflet pairs than those with tl alone; thus, in this respect, tac is

epistatic to tl. In an af_ background the distinction between tac tl and

tac Tl/- is unmistakable. Moreover, the difference between af tac tl and

af Tac tl is evident by the larger and fewer laminae in the former than in

the latter. (This difference evidently was also recognized by Sharma (7)

because, in his diagramatic illustrations, he depicts the leaflet size of

af tac t1 plants as larger than af Tac t1 plants). Thus, tac is an inter-

esting and powerful mutant gene in a number of respects.

There remains the question of symbolization. Because the discovery

of unifoliata (1) antidates tendriled acacia, perhaps the symbolization

should be: Uni,Uni^tac ,uni. According to Sharma and Kumar (8) there already

exist two alleles of tac: tac^b , and tac^s . Whether Uni is a classical

multiple allelic locus or a complex locus remains to be seen.

1. Lamprecht, H. 1933. Hereditas 28:269-296.

2. Lamprecht, H. 1948. Agri Hort. Genet. 6:10-48.

3. Marx, G. A. 1972. PNL 4:30-31.

4. Marx, 6. A. 1974. PNL 6:30-31.

5. Marx, G. A. 1984. PNL 16:46-48.

6. Marx, G. A., N. F. Weeden, and R. Provvidenti. 1985. PNL 17:57-60.

7. Sharma, B. 1981. Pulse Crops News1 . 1(1):56-57 .

8. Sharma, B. and S. Kumar. Pulse Crops Newsl. l(3):2 1-22.



	PNL Volume 18 1986 RESEARCH REPORTS	51

	Table 1. Analysis of F₂ populations derived from the cross A M Tac Apu St x A m tac apu st.


	Table 2. Analysis of F₂ populations derived from the cross Tac Apu St wel x tac apu st Wel.



	52 PNL Volume 18 1986 RESEARCH REPORTS

	Table 3. Analysis of populations derived from three crosses of the constitution: apu tac x Apu Tac,



	Table 4. Analysis of three F2 populations derived from three-point crosses (a) st apu tac x St Tac St, (b) St_ a£u tac x st Apu Tac, and (c) st Apu Tac x St apu tac.