INBREEDING DEPRESSION OF SEVERAL INTRODUCED POPULATIONS OF MAIZE (Zea mays L.)

TITLE:

INBREEDINGDEPRESSIONOFSEVERALINTRODUCEDPOPULATIONSOF MAIZE(ZeamaysL.)

AUTHOR:

R.T.VIANNA

EE.GOMESEGAMA

V.NASPOLINIFILHO

JR.MÔRO

R.VENCOVSKY

INBREEDING DEPRESSION OF SEVERAL INTRODUCED POPULATIONS. OF MAIZE (Zea mays L.)

R.T. VIANNA, EE. GOMES : GAMA, V. NASPOLINI FILHO, JR. MORO, R. VENCOVSKY

Centro Nucional de Pesquisa de Milho e Sorgo, Caixa Postal 151, 35700, Sete Lagoas, MG, Brasil -

Received January 21, 1982

ABSTRACT - An inbreeding depression study was conducted with 14 maize populations introduced from CIMMYT to the National Corn and Sorghum Research Center, Sete Lagoas, MG, Brasil. It was initinced by selfing 200 unselected S, plants per population and from each 100 ears were seleczed. The S, generation of each population was represented by a mixture of 100 kernels from each of the 100 selected ears. Experiments were conducted in 1979 at two locations to evaluated yield change associated with inbreeding by selfing. A drought period caused considerable yield reduction at Piracicaba. In these populations, with inbreeding due to only one selfing generation, mean yield reduction ranged from 31 to 65% and 44 to 69% at Sete Lagoas and Piracicaba, respectively. In both locations a significant interaction between population and inbreeding depression iwas observed. [n general materials grown under moisture stress did not respond the same as under non stress conditions. In the non stress environment data confirmed that the synthetic variety showed a significantly lower igbreeding depression than broad basis populations.

KEY WORDS: Zea mays L.; Inbreeding depression; Population; S, line; Yield. e

INTRODUCTION

The theory of inbreeding was developed by WRIGHT (1922) and MaLEcOT (1948). SHuLL (1908), EAsT (1908) and JoNEs (1924) describe the general consequences of inbreeding and suggested the potential use of inbred lines of maize for the development of hybrids.

Just._as heterosis is the result:of mating unrelated individuals, inbreeding depression is the result of mating- related individuals. In most cross-fertilizer species' self-fertilization results in drastic inbreeding depression. This: phenomenon causes the loss: of many inbred lines under development. Thé development of productive maize inbred lines is possible if vigor and yield are: maintained as inbreeding progresses towards _homozygosis.

Differences in rates of inbreeding depression among populations would be expected because of differences in gene frequency and level of dominance

(HALLAUER and SEARs, 1973). Depression itself is due to reduction in the amount of heterozygosis present asscciated with some level of dominance. On the other hand epistasis, an interallelic interaction, has not been shown to play as great a role in inbreeding depression. The presented statistical models to measure epistasis are, however, insufficient as shown by SILVA and HALLAUER (1975). To single out a germplasm for developing pure lines it is very important to know its reaction to inbreeding depression if success for use in producing maize hybrids is desired.

GARDNER (1965) suggested the use of a genetic parameter (i + @) to better identify a population for inbreeding work. An example was presented where expected yields of lines were very low, but did agree with experimental results.

This study was conducted to evaluate the inbreeding depression effects in several introduced maize populations to be used as a source for extracting inbred lines.

MATERIAL AND METHODS

Fourteen maize populations with intermediate plant height not adapted to moisture stress, introduced from CIMMYT-MEXICO, were used (Table 1). Populations 01, 03, 06 and 08 are germplasms under recurrent selection, originated from the intercrossing of selected full-sib families. On the other hand, populations 02, 05 and 07 are advanced generations of an intervarietal cross. Populations 09 to 14 are composites of broad genetic basis. Each one was obtained by the intercrossing of several germplasms, similar in relation to agronomic characteristics, Population 04 is a synthetic developed thru selection and recombination of 16 S, lines, and has a narrow genetic base. During its synthesis, selection "against deleterious recessive genes probably produced a decrease in its genetic load which accounted for a low level of inbreeding depression. The opposit occurs with composites wich have a broad genetic base and consequently heigher frequencies of deleterious recessive genes or a greater genetic load, and therefore can tolerate inbreeding depression less than synthetic.

Approximately 200 vigorous plants in each of the 14 populations were self-pollinated in 1978 and 200 S,\;ars were selected with sufficient seed to plant two replicate yield trials. For the vield trials, which included S, (populations) and §, (self-pollinated) entries, a sample of kernels of the populations and a bulk of 100 kernels of each S, line were employed. Each trial was grown in a randomized block design in strips, with five replications in each of two locatipns, Sete Lagoas (Minas Gerais) and Piracicaba (Sio Paulo), Brasil. During the growing season there was a drought period at Piracicaba and irrigation could not be provided to suplement natural rainfall.

Due 10 low yields at Piracicaba brought about by drought, in addition to the Jarge

difference of error. mean squares between locations, analyses were made on a location basis and not for combined locations: Hence, results were mainly based on location analysis and yielding data of Sete Lagoas.

Entries in both trals were grown in tworow plots, 5m long, 0.75m apart, with two plants per-hill spaced 40 cm apart in the row, providing 66.600 plants/ha.

Inbreeding depiession was calculated on the basis of average grain yield of S; and S, plants, of each maize populaticn, within trials.

Components of S, populations means were also estimated according to GARDNER (1965). The model, wich assumes randon mating parental populations and negligible epistasis, is such that an S; mean may be partitioned into 3 + a and d, respectively the contribution of homozygous and heterozygous loci to the mean. Here i + a is the average expected mean of randon inbred lines, extracted from the population.

RESULTS AND DISCUSSION

The analysis of variance for yield is given in Table 2. The C.V.s. at Sete Lagoas (MG) were within normal acceptable levels, however at Piracicaba (SP) higher values were observed mainly due to a long dry period in critical developmental stages. The mean squares of populations (P) and inbreeding (I) were highly significant, in both locations. The mean squares for

(') Origin = CIMMYT

(3) TL = Tropical Lowland; TE- = Temperate; Y = Yellow Kernell; D = Dent; SD = Semi-Dent; F = Flint; SF = Semi-Flint; LM = Late Maturity; IM = Intermediate Maturity; EM = Early Maturity.

TABLE 1 - Introduced maize populations used to study the effect of inbreéding depression. *

GOMES ¢ GAMA, V. NASPOLINI FILHO, JR. MORO, R. VENCOVSKY

the interaction of populations with inbreeding (P xI) were also significant, indicating difference of reaction of population to inbreeding effect. Mean yield, mean inbreeding depression and estimates of genetic effects, for each population grown in each of the two yield trials, are given in Table 3. The average inbreeding depression (over two locations) was higher in S material from pools (52%) than in the $ from other populations (46%), but the difference was not statistically significant. Greater inbreeding depression is expected in pools if the diversity of germplasm going into the pool is such that the number of heterozygous loci is greater than in other kind of populations. .

Mean yield reduction, due to one generation of selfing, ranged from 31 to 65% in the non moisture stress condition (Sete Lagoas). In this invironment, data confirmed that a synthetic variety (Suwan DMR), derived from s lines, showed less inbreeding depression than broad basis populations. Under the moisture stress environment (Piracicaba), inbreeding depression ranged from 44 to 69%.

In relation to both environments, some populations were less stable than others as shown by the inbreeding depression effect of the S; material. For example, the Suwan DMR clearly showed a lower inbreeding depression effect (31%) at the non moisture stress condítio;z, and higher (57%) in the other environment. This agrees with FALCONER's study (1960) where he asserted that inbred individuals often show more non-genetic variation that non inbred individuals. Also, it was observed that the inbreeding effect was great for populations grown in the moisture stress environment. However, three populations: Pool 33, Eto x Illinois and Amarillo Subtropical showed a slightly Jower inbreeding effect in the moisture stress environment.

The expected mean yields for random inbred lines (i + o) of the 14 populations are also shown in Table 3. Estimates of these expected mean yields, in the moisture stress environment, were mostly negative or very low, indicating low probability of success of selecting inbred material in this condition. Considering the non stress environment (Sete Lagoas), as more reliable for breeding purposes, the expected mean yields of random inbred lines ranged from-990 to 1880 kg/ha. It seems that Suwan DMR is the best source population for extraction of inbred lines with better performance per se. For most pbpulations, the contribution for heterozygous loci (d) to the population mean was considerably high.

Also, was shown by VENcOVSKY (1978), a contrast of the type (u + a)

*, ** Significant at the 0.05 and 0.01 levels of probability, respectively.

The authors wish to thank Dr. Anténio Carlos de Oliveira for his statistical assistance.

(B2 + @) measure the expected difference in general combining ability effects of populations 1 and 2, assuming that both were test crossed to a tester population with allelic frequency equal to 0.5 at all loci.

If a synthetic variety is a combination of inbred lines, then less deleterious recessive genes would be expected to be segregating, resulting consequently in a lower inbreeding depression effect. HALLAUER and SEars (1973) working with a synthetic variety of maize, BSSS, reported an inbreeding depressicn effect for yield of 31% from the S to the S, generation. However, as indicated in Table 3, some populations with broad genetic basis showed lower inbreeding effects. It is believed that differences in gene frequency and levcls of dominance are responsible for observed difference in rates of inbreeding depression among populations. The Suwan DMR showed the -rlowest inbreeding depression value at!Sete Lagoas, however it did not repeat its performance under stress condition (Piracicaba).

- The results do bear out the hipothesis that synthetic varieties would have- a significantly lower inbreeding effect than a broad genetic basis populaticn, in non stress. environments. On the other hand, tolerance to inbreeding depression of these ssynthetic varieties; in moisture stress environ- ments, can be reduced indicating a probable loss or sensible decrease of frequency of specific genes or gene complexes responsible for this tolerance, due to genetic basis reduction.

TABLE 2 - Analysis of variance for mean yiíeld (kg/ha) of twa locations, 1978-1979.

INBREEDING DEPRESSION 157

RIASSUNTO

Depressione da inbreeding in popolazioni introdotte di mais

E' stato condotto uno studio sulla depressione da inbreeding con 14 popolazioni introdotte dal CIMMYT alla sazione nazionale brasiliana per il mais e il sorgo. Sono state auto- fecondate 200 piante S, non selezionate per ogni popolazione. Da queste si sono campionate 100 spighe. La generazione S, per ogni popolazione era rappresentata da una mistura di 100 semi per ognuna delle cento spighe. Sono stati poi condotti nel 1979 due esperimenti per valutare la caduta produttiva dopo autofecondazione. À una localita un periodo di assenza di pioggia ha causato una considerevole riduzione della produzione. La riduzione media & variata dal 31 al 65% in una localita e dal 44 al 69% nell'altra. Si & anche osservata in entrambe le localitã una interazione significativa tra le popolazioni e la depressione da inbreeding. In generale i materiali allevati in condizioni di stress non hanno risposto nello stesso modo come quelli allevati in condizioni normali. In questa ultima situazione i dati hanno confefmato che le varietà sintetiche hanno una depréssione da inbreeding meno drastica di quella delle varietd con larga base genetica.

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