Transgressive segregation is the formation of extreme phenotypes observed in segregated hybrid populations compared to phenotypes observed in the parental lines. Genetic causes of transgressive segregation such as increased mutation rate, exposure of recessive alleles, over dominance, breakdown of linkage, complementary gene action and epistasis plays vital role in understanding the genetic basis of transgressive segregation (Nirubana et al., 2021). Based on the classical explanations through the “Omnigenic theory for quantitative traits” certain new combinations of core and peripheral genes contribute to transgressive segregation which is a resultant of novel biochemical or physiological effects.

Mao et al., (2011) observed transgressive segregation for kilo-grain weight (KGW) in rice recombinant inbred line (RIL) population derived from the cross Teqing and 02428. They showed that complementary action of additive alleles was an important genetic factor for transgression along with positive digenetic interactions and epistasis. Koide et al., (2019) studied genetic properties responsible for the transgressive segregation of days to heading (DTH) in rice in F2 population derived from a cross A58 x Kitaake with proximal DTH. They revealed that transgressive phenotypes were produced by exchanging complementary alleles of a few minor QTL in the similar parental phenotypes. Many of the parental QTL alleles that showed non-epistatic positive complementation in transgressive segregants had opposite phenotypic effects in their parental genetic background (Benildo et al., 2019). Genome shock could have important consequences at the epigenomic level by causing either localized or global changes in DNA methylation landscape, and even histone landscape which contributes to transgression. Shivaprasad et al., (2012) suggested the possibility of association of transgressive expression of micro or small interfering(si) RNA loci in segregating generation of cross between Solanum lycopersicum cv. M82 and Solanum pennellii, accession LA716 due to epigenetic phenomenon called hyper methylation.

Transgressive segregants play important role in adaptive evolution due to their unique advantages under ecological niches. Current evidence based on abundance of genetic and genomic data supports the theory that transgressive segregation has both genetic and epigenetic components, either in conjunction with each other or independent from eacsh other. A new breeding paradigm must be geared towards elucidating and selecting for complex interactions in the genome and epigenome that contributes transgressive segregation.

References:

BENILDO, G. D., 2019, Genomic and epigenomic bases of transgressive segregation: New breeding paradigm for novel plant phenotypes. Plant Sci., 288:110213.

KOIDE, Y., SAKAGUCHI, S., UCHIYAMA, T., OTA, Y., TEZUKA, A., NAGANO, A. J., ISHIGURO, S., TAKAMURE, I. AND KISHIMA, Y., 2019, Genetic properties responsible for the transgressive segregation of days to heading in rice. G3: Genes, Genomes, Genetics, 9(5):1655-1662.

MAO, D., LIU, T., XU, C., LI X AND XING, Y., 2011, Epistasis and complementary gene action adequately account for the genetic bases of transgressive segregation of kilo-grain weight in rice. Euphytica, 180:261-271.

NIRUBANA, V., PALANIYAPPAN, S., KARTHIKEYAN, M. AND DEEPIKA, C., 2021, Transgressive Segregation and Its Importance in Crop Improvement.

SHIVAPRASAD, P. V., DUNN, R. M., SANTOS, B. A., BASSETT, A. AND BAULCOMBE, D. C., 2012, Extraordinary transgressive phenotypes of hybrid tomato are influenced by epigenetics and small silencing RNAs. The EMBO J., 31(2):257-266.