Identification of essentially derived varieties obtained from biparental crosses of homozygous lines: II. Morphological distances and heterosis in comparison with simple sequence repeat and amplified fragment length polymorphism data in maize

In addition to genetic distances (GDs) based on molecular markers, morphological traits and heterosis have been proposed as possible tools to assess the genetic conformity between putative essentially derived varieties (EDVs) and their initial varieties (IVs). However, for maize (Zea mays L.) and other crops, no consensus has been reached regarding methods and thresholds for identification of EDVs, maiuly because reliable benchmark data are lacking. The objectives of this study were to (i) evaluate the power of morphological traits and heterosis to discriminate between homozygous progenies derived from F ₂, BC₁, and BC₂ populations (BC = backcross), (ii) compare the findings to published data based on simple sequence repeats (SSRs) and amplified fragment length polymorphisms (AFLPs), and (iii) draw conclusions about the usefulness of the various criteria for identification of EDVs. Morphological distances (MDs) based on 25 traits and midparent heterosis (MPH) for 12 traits were observed for a total of 58 European maize inbred lines comprising 38 triplets. A triplet consisted of one homozygous line derived from a F₂, BC₁, or BC₂ population and both parental inbreds. In addition, all inbreds were genotyped with 100 uniformly distributed SSR markers and 20 AFLP primer combinations in companion studies for calculation of GDs. Correlations between the coancestry coefficient, GDs, MDs, and MPH were significant and high for the majority of traits. However, thresholds for EDVs to discriminate between F₂- and BC₁-derived, or BC ₁- and BC₂-derived progenies using only MDs or heterosis yielded considerably higher values for Type I (α) and Type II (1 - β) errors than observed with GDs based on SSRs and AFLPs. Consequently, morphological traits and heterosis are less suited for identification of EDVs in maize than molecular markers.