Doubled haploid versus S₁ family recurrent selection for testcross performance in a maize population

Abstract

Theoretically, in a recurrent selection program, the use of doubled haploids (DH) can increase genetic advance per unit of time. To evaluate the efficiency expected from the use of DH for the improvement of grain yield in a maize (Zea mays L.) population, two recurrent selection programs for testcross performance were initiated using testcross progenies from DH lines and S₁ families. In 4 years one selection cycle using DH and two selection cycles using S₁ families were carried out with the same selection intensity for both methods. As expected, testcross genetic variance was twice as high among DH lines as among S₁ families. The predicted genetic gain was 8.2% for the DH selection cycle, and 10.6% for the two S₁ selection cycles, giving a per year advantage of 29% for the S₁ family method over the DH method with a cycle of 4 years. With a 3-year cycle for the DH method, both methods were expected to be equivalent. Using a tester related to the one used for selection, the genetic gains obtained were equivalent for both methods: 6.6% for the DH cycle and 7.0% for the two S₁ cycles. With a 3-year cycle for the DH method, the advantage would have been in favor of DH method. Furthermore, the DH method has the advantage of simultaneously producing lines that are directly usable as parents of a hybrid. Thus, if the genetic advance per unit of time is evaluated at the level of developed varieties even with the same or with a lower genetic advance in population improvement, the DH method appears to be the most efficient.

Appendices

Appendix 1: derivation of the coefficient of inbreeding of the C1DH population

Among the 52 intercrossed DH lines used to develop the C1DH population, 28 derived from independent S₁ with one DH line per S₁, 18 derived from nine independent S₁ with two DH lines per S₁ and 6 derived from two independent S₁ with three DH lines per S₁.

The coefficient of inbreeding is defined as the probability of drawing at a locus in a zygote two genes identical by descent, i.e., deriving from the same ancestor gene. Assuming random mating, identity by descent in one zygote can result either from selfing with a probability of 1/52, or by crossing between sister lines, i.e., from the same S₁. In the case of crossing between two sister lines, the expected inbreeding coefficient of progeny is1/2. One S₁ with two sister lines generates two crosses between sister lines (including reciprocal); one S₁ with three sister lines generates six crosses between sister lines. The total inbreeding coefficient will thus be:

$$ 1/52 + 9{\text{ }}(2{\text{ }}\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} )1/522 + 2{\text{ }}(6{\text{ }}\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} )1/522 = 0.0247. $$

Appendix 2: genetic advance in varietal development

Total genetic advance with varietal development (ΔG _TV) is derived by adding genetic advance due to varietal development from one selection cycle of population improvement (ΔG _V) and the cumulated genetic advance due to population improvement (ΔG _P). Assuming linear response over the first cycles of selection, the expression of total genetic advance after n cycles of recurrent selection followed varietal development is

$$ \Delta G_{{{\text{TV}}}} = n\Delta G_{{{\text{P1}}}} + \Delta G_{{\text{V}}} , $$

where ΔG _P1 is the genetic advance in one cycle of population improvement. Such a genetic advance needs a time t=n c _P+c _V, where c _P is the cycle length in population improvement (3 or 4 years for the DH method) and c _V is the duration of varietal development. Therefore, genetic advance per unit of time can be derived.

To consider varietal development with the S₁ method, several schemes are possible. As this is not the place to discuss all the schemes that are possible, we simplified by considering SSD from the best S₁ and assumed that the same potential can be achieved as with DH. Using off-season nurseries, S₅ or even S₇ lines can be derived in 2 years with 1 year more for the evaluation of testcross progenies (which can begin at the S₄ level). This gives a minimum duration of 5 years after the population resulting from intercrossing. The same formula as previously can then be used with appropriate parameters. Obviously, it is not justified to study more than 3–4 cycles without considering the possible decrease in genetic variance. It should be noted that 5 years is the minimum time for the derivation of lines without the use of DH; such a value favors S₁ method.

Figure 1 shows the comparison of genetic advance per year for both methods with the following conditions:

• the potential of varietal development for DH selection was computed by selecting the best 3% lines;

• genetic variance among DH lines was taken to be equal to 44 which is the value estimated at the first DH selection cycle and which is quite consistent with variance among S₁ estimated at the end of the experiment;

• heritability at the level DH lines was taken to be equal to 0.80: it was estimated to be equal to 0.83 in the first selection cycle. Genetic advance due to varietal development ΔG _V from any selection cycle was then computed as i h ² σ_G=11.9 q ha⁻¹; it should be noted that, with the assumption of the same potential for both methods in varietal development, such a quantity does not affect the difference between the two methods;

• for the genetic advance due to population improvement, expected and realized values were taken into consideration.