Tester stocks for B-A translocations

--J. B. Beckett

Both seedling and endosperm recessive traits are useful for identifying male parents that carry intact B-A translocations (i.e., plants that carry both the A-B and B-A chromosomes, and thus can support nondisjunction of the B-A chromosome; an exception, TB-10L18, is discussed below). In a tester stock, the presence of endosperm and seedling traits in coupling is advantageous because, in testcrosses, 1) most good males can be identified immediately by the "segregation" of the recessive endosperm phenotype, 2) ears with scant seed set can be tested further by observing for recessive seedling phenotype in the sandbench and 3) because accidental contamination, usually selfing and sibbing within the tester stock, can be detected more readily (since testcross kernels that have the recessive endosperm phenotype should never produce seedlings with the recessive seedling phenotype of the other traits involved).

Loci used to detect the presence of a B-A translocation obviously need to be beyond the translocation breakpoint. If more than one translocation on a chromosome arm is being tested, at least one locus must be beyond the most distal translocation. In general, recessive alleles of seedling and endosperm traits should be in coupling. If not viable, two markers in repulsion can be selected to be as close together as possible so that they can be maintained as a balanced stock by selfing and eliminating the recessives each generation.

Even when recessive alleles are in coupling and are fully viable, a single stock is inadequate because vigor declines until the stock is not dependable as a female parent. Introgressing the recessive genes into two backgrounds allows the production of vigorous F1 testers.

If two such stocks are not available, suitable testers can be produced by pollinating inbred lines or appropriate F1 standards by the tester stock. Although the apparent frequency of nondisjunction will be cut in half, the likelihood of determining whether the male parent is valid or not is much greater when the tester stock is hybrid and vigorous.

When both of the recessives are lethal and relatively close together, balanced lethal tester stocks (carrying the two genes in repulsion) are excellent except for the fact that, again, the stocks become inbred and make poor seed parents. Outcrossing balanced lethals to inbred lines or standards generates usable testers, although each tester plant carries only one of the two recessives. Obviously, if the loci are too far apart, neither the balanced lethal stock nor its outcrosses will be fully dependable as testers.

Below are listed some of the testers that are in current use in my laboratory: (Symbols used: std = standard, such as a hybrid between two inbreds. An appropriate inbred can be substituted. Y std = yellow standard; W std = white standard; R-sc std = C R-sc or C R-scm standard; njW23 = A C R-nj version of W23. cl Ps = colorless endosperm, colored embryo. Parenthetical numbers, such as W23(4), indicate the number of times the tester stock has been crossed, not backcrossed, to the inbred. Stocks with unknown or complex backgrounds are designated "unk". Bcgd = background. m/c = may carry.)

TB-1Sb std x [(+ dek1)/(nec2 +)]

Male is a fairly stable balanced lethal that is maintained by selfing. Bcgd unk. If the std is C R-sc, or if the TB parent carries C and R-sc, at least some of the dek kernels produced from crossing by TB-1Sb will be cl Ps. TB-1L's bz2 x bz2 bz2 x an1-6923 (an-6923 gives bz kernels)

R-sc std x bz2

The hypoploid seedling phenotype is so obvious that a seedling trait is unnecessary.

Bcgds: bz2 C R stocks: W23(4)(from E. H. Coe); K55(4)(from Coe).

+/an1-6923 x an1-6923, C R (or R-sc) 2 stocks, unk and njW23(3), respectively.

 TB-2Sb and TB-3La-2S6270 Y std x [al1, (+ +)/(lg1 gl2)] The al was selected from [(+ + +)/(al1 lg2 gl2)] selfed. A (+/al1, lg1 gl2) x al1 lg1 gl2 stock would be better. Craig Echt's TB-2Sa, which carries B-Peru on the B-2S chromosome, is still being confirmed. If it is a good translocation, it will give cl Ps kernels when crossed onto b b C C r r tester, preferably carrying gl2 or d5. Bcgd of al stock unk. TB-2L's std x [(v4, +/w3, +/spt1) selfed] w3 and spt1 are needed for TB-1Sb-2Lc, but they are too far apart to produce a stable balanced lethal stock. Bcgd unk. TB-3Sb Y std x d1 cl1 Clm-4 Select dwarfs from a +/d1, cl1 Clm-4 x d1 cl1 Clm-4 stock. Bcgd unk.

TB-3L's Various F1 and 3-way crosses involving the stocks listed below, and sometimes R-sc std, are used.

Stocks and bcgds: a1-m C R-scm2 (unk)(from J. Birchler and K. Newton)

gl6 lg2 a1 et Dt1 C R (unk); gl6 lg2 a1 Dt1 C R-sc (86.3% L289)

 TB-4Sa std (or su1) x (+ hcf23)/(su1 +) std (or su1) x (su1 +)/(+ bt2)

The hcf/su balanced tester stock is maintained by selfing and discarding su; hcf23 is a near-white lethal. The su/bt balanced stock is maintained by selfing and discarding su and bt grains. Bcgds: hcf/su (unk); su/bt (unk). A +/su1 x su stock (98% L289) is available.

 TB-4L's R-sc std x gl3 c2 (and preferably dp1) C1 R-sc R-sc std x gl3 c2 C1 sh1 R-sc

F1's involving this sh1 stock make good testers for TB-9Sb-4L6222 and TB-9Sb-4L6504.

Bcgds: (gl3 c2, +/dp1, C1 R-sc) selfed (75% W23)

(gl3 c2 C1, +/sh1, R-sc) selfed (25% W23, 12.5% L289)

 TB-5S's R-sc std x gl17 a2 R-sc std x a2

Y std x (+ ps1)/(vp2 +)

Stocks and bcgds: [(+ ps1)/(vp2 +)] selfed (unk)

gl17 a2, +/bt1 x bt sib, prob R-sc (74% L289) m/c pr

gl17 a2, +/bt1, +/v2 x bt, +/v2 sib (75% njW23) m/c pr

[(a2 + + + pr)/(a2 bm1 bt1 bv1 pr); C R] x a2 bm bt bv sib (unk)

The (ps/vp) selfed stock is a balanced lethal; it can be used with TB-1La-5S8041 but not TB-5Sc. bm, an adult plant trait, is uncovered by TB-1La-5S8041 but not by TB-5Sc. (See 5L testers below).

 TB-5L's pr1 gl8 C R-sc W23(2) x pr1 gl8 C R-sc L289(2) R-sc std x pr1 gl8 C R-sc

R-sc std x [a2 bm1 pr v2 C R (25% N6)]

bt is not uncovered by any TB-5L; bv is an adult plant trait. (See 5S testers above).

 TB-6Sa W or Y std x hcf26 y1 Most TB-6La stocks carry y1 on the 6-B chromosome, so the presence of y in the tester is useful. A (+ dek28)/(hcf26 +) balanced tester is being developed.

Bcgds: (+/hcf26 y1)/(hcf y) x hcf y (12.5% W23, 22% 38-11); gl8 gl8

(+ Y)/(hcf26 y1) x hcf y

4 stocks, all 6% W23 and 11% 38-11, in 50% Mo17Ht, A632Ht, B73Ht and L317 bcgds, resp. All seg gl8.

(+ Y)/(hcf26 y1)] selfed (30% W23, 11% 38-11) Seg gl8

(+ Y)/(hcf26 y1)] selfed (56% W23) Seg gl8

hcf26 Y1 selfed (25% 38-11, 25% WF9)

 TB-6L's W std x y py (or Y py) W std x yellow kernels from [(+ Y)/(l12 y1)] selfed

The white kernels of W std x (+ Y)/(l12 y) produce the best testers.

Use std x py to test TB-6Lb.

Bcgds: l12 stock (unk)

y1 pl1 sm1? py1 selfed (44% WF9) WF9 gives unusually tall py plants

(Y Pl sm1 py1)/(Y Pl sm py)/(Py) x py sib (56% Oh51A); P-RR

The + allele of py is carried on a B-6L chromosome from TB-6Lb

 TB-7Sc Y std x vp9/vp9/+ The vp stock, which is 25% L289, is maintained by selfing, since vp9 is lethal. The + allele of vp9 is carried on an apparent telocentric chromosome. The telosome, which is transmitted poorly, depresses kernel size. o2, which has been hard to recognize in my stocks, has been ignored. A balanced stock, (v5 +)/(+ vp9), could be maintained readily by selfing because adult v5 plants are striped and thus can be eliminated. Unfortunately, v5 is not a dependable sandbench trait. TB-7Lb Y std x o5 gl1 The o5 stock maintained as +/o5, gl1 x o5 gl1 sib (50% L289). TB-8Lc std x normal grains from (pro/pro/+)selfed std x normal from (pro +)/(+ v21)/(+ +)]selfed

A B-8 chromosome from TB-8Lc carries both + alleles. The stock is not stable because pro and v21 are not closely linked; a [(pro v)/(pro v)/(+ v)] selfed derivative, if viable, would be better. Bcgd unk.

 TB-8La and TB-8Lb std x v21 The tester is usually carried as +/v21 x v21. Bcgd unk.

TB-9S's R-sc std (or c1 R-sc std) x yg2 c1 sh1 wx R (or R-sc)

Bcgds: One yg2 stock is 75% njW23; another is 25% W22, 25% W23

The seedling trait d3 is excellent but hard to use because dwarf plants shed little pollen. Neither wx nor d3 is uncovered by TB-9Sb.

TB-9L's Y std is the tester for homozygous TB-9Lc (Wc Wc)

Hypoploid endosperms are yellow and somewhat smaller than normal. The homozygous TB-9Lc stock is in undefined background.

Std x c1 wx v1 gl15 Bf1 bm4

Bcgd of gl15 tester: 12.5% Oh45, 12.5% Oh51A

Screen for gl15 at the 3 to 4-leaf stage; v1 can usually be recognized in a cold sandbench. Only Bf1 is uncovered by TB-9La; use an appropriate ultraviolet light and goggles.

TB-10Sc Y std x y9

Std x oy

The y9 stocks are [(sr3 +)/(+ y9)] selfed and (+/sr3, y9) selfed, but the sr3 plants usually die before flowering. The oy stock is maintained as oy selfed or +/oy x oy. Unk bcgd for all testers.

TB-10L's g1 r1 C std x g1 r1 C

C r std (or R-sc std) x g1 r1 C

Three g1 r1 C stocks are available; the 3-way cross is rather vigorous. The TB parent should carry R-sc so that cl Ps kernels will be produced. I use these testers for all TB's on 10L; the presence of cl Ps kernels on testcross ears is generally diagnostic. For several of the more proximal TB's, cl Ps kernels are often smaller than normal, thus helping confirm the presence of intact translocations. For TB-10L18, with the B chromosome breakpoint in the short arm, testcrosses should be checked for golden hypoploids in the sandbench, or cl Ps kernels on testcross ears must be clearly smaller than normal; otherwise, the 10-B chromosome will soon be lost. To classify for golden in the sandbench, cut seedlings off just above the coleoptilar node and examine the cut ends for green vs. golden. Striate-2 is not a dependable seedling marker.

Stocks and bcgds: (C + r-r)/(C g1 r-g) x C g1 r-g (L289(4))

C g1 r-g (81% njW23)

C g1 r-r (?% W22)

Stocks used to make up standards, etc.:

C R-sc L289(10) y1 c1 L289(8)

C R-scm L289(8) y1 C-I L289(6)

C r-r L289(10) y1 Hy2(4)

c1 R-sc L289(10) y1 38-11(5)

c1 R-scm3 L289(8)

C R-nj W23 (= njW23) Originally obtained from Devender Nanda; the number of backcrosses to W23 is unknown; the stock is much like W23 but the pericarp is transparent instead of dull.

C R-sc njW23(11) The hybrid with R-sc or R-scm L289 = R-sc std.

C R-scm njW23(4)

c1, +/sh1, R-sc x sh sib (L289(9))

C r Y inbreds: FR35, N28Ht, H100, 38-11

C r y1 inbreds: K44, K61, Mo16W, Ky27, Ky228 Some or all of these may carry Wc.

 Seed of the stocks described above is available in small quantities upon request.
Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors

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