Balanced lethal systems
In evolutionary biology, a balanced lethal system is a situation where recessive lethal alleles are present on two homologous chromosomes.[1] Each of the chromosomes in such a pair carries a different lethal allele, which is compensated for by the functioning allele on the other chromosome.[1] Since both these lethal alleles end up in the gametes in the same frequency as the functioning alleles, half of the offspring, the homozygotes, receive two copies of a lethal allele and therefore die during development.[1] In such systems, only the heterozygotes survive.[1]
Balanced lethal systems appear to pose a challenge to evolutionary theory, since a system so wasteful should be rapidly eliminated through natural selection and recombination.[2] Instead, it has become fixed in various species all over the tree of life.[1][2]
Mechanism
The exact mechanism behind balanced lethal systems remains unknown. Prior to the availability of efficient DNA sequencing methods, it was already known that the lethality in such a system was caused by homozygosity of a certain chromosome pair.[3][4]
One theory is that, in the case of the Triturus genus, the balanced lethal system is a remnant of an ancient sex-determination system.[1][2] One of the chromosomes of the pair that contains the system is longer than the other, which is also the case for the actual sex chromosomes.[2] In this theory, deleterious mutations accumulated on the non-recombining part of the Y-chromosome (Muller’s ratchet).[1][2][5] Then, two distinct Y-chromosomes, both with different lethal mutations, co-segregated in a population.[1] Since sex-determination in many cold-blooded vertebrates is potentially dependent on temperature, a shift away from chromosomal sex determination occurred.[1][2] This system favoured the sex reversal of females, which eventually led to the loss of the original X-chromosome.[1][2] A mutation on another chromosome later restored the even sex ratio, and gave rise to a new male-heterogametic system.[1][2] A major restriction for this theory is that it could only evolve in species where temperature-dependent sex-reversal is possible.[1] Since balanced lethal systems are found in many species where this is not the case, this theory does not provide a general explanation for how such a system evolved.[1]
Another theory is that balanced lethal systems are collapsed supergenes.[1] Supergenes are linked genes that are inherited as a single unit.[6] Genes can only be inherited together when recombination is suppressed, for example when selection favors certain allelic combinations.[6] The lack of recombination can lead to the accumulation of mutations in both supergene clusters[1][6] and this could generate a feedback loop:[1] when natural selection favours heterozygotes, few homozygotes reproduce.[1] This lack of reproduction leads to the accumulation of deleterious alleles. When lethal mutations become fixed on both supergene alleles, homozygotes are no longer viable, resulting in a balanced lethal system.[1]
Prevalence
A well known balanced lethal system is the one fixed in the genus Triturus (containing the crested and the marbled newts).[4] Each of the homologous chromosomes of pair 1 (1A and 1B) has a different recessive deleterious allele on a non-recombining section of the chromosome.[2] Therefore, only heterozygotes are viable since these deleterious alleles are compensated for by the functioning allele on the other homologue. As a result half of all offspring stop growing and die during early development.[2]
The offspring of Triturus carnifex for example, have either a viable heterozygous genotype (1A/1B) or one of the homozygous embryonic lethal genotypes: fat-tailed (1A/1A) or slim-tailed (1B/1B).[7]
See also
- Balancer chromosome
- Recessive lethals
- Northern crested newt
- Hermann Joseph Muller
- Linkage disequilibrium
References
Further reading
- Azevedo JL, Roper JA (October 1967). "Lethal mutations and balanced lethal systems in Aspergillus nidulans". Journal of General Microbiology. 49 (1): 149–55. PMID 6064441.
- Schwander T, Libbrecht R, Keller L (March 2014). "Supergenes and complex phenotypes". Current Biology. 24 (7): R288–94. S2CID 15829802.
- Muller HJ (September 1918). "Genetic Variability, Twin Hybrids and Constant Hybrids, in a Case of Balanced Lethal Factors". Genetics. 3 (5): 422–99. PMID 17245914.
- Yan Y, Mu W, Zhang L, Guan L, Liu Q, Zhang Y (May 2013). "Asd-based balanced-lethal system in attenuated Edwardsiella tarda to express a heterologous antigen for a multivalent bacterial vaccine". Fish & Shellfish Immunology. 34 (5): 1188–94. PMID 23454428.
- Dawson PS (August 1967). "A balanced lethal system in the flour beetle, Tribolium castaneum. Heredity". 22 (3): 435–8. )