Segregation distorter (SD)

Definition

A selfish genetic element in heterozygotes that subverts Mendelian segregation to transmit itself to more than half of the functional gametes, typically by destroying or disabling competing chromosomes. SD systems exploit molecular mechanisms—often involving satellite DNA, small RNAs, or toxin–antidote systems—to bias transmission without conferring fitness benefits to the organism. The classic Drosophila melanogaster SD complex on chromosome 2 includes a responder (Rsp) locus with repetitive DNA that is targeted for dysfunction by the distorter, causing non-Mendelian inheritance ratios.

Etymology

From Latin segregare (to set apart) + distorquere (to twist or distort), reflecting the deviation from equal segregation of alleles.

Example

In Drosophila melanogaster, males carrying the SD factor on chromosome 2 transmit it to over 95% of progeny because the distorter disrupts sperm carrying the sensitive responder (Rsp^s) allele, leaving only SD-bearing sperm functional; this has driven the rapid evolution of suppressor loci in natural populations.

Synonyms

• meiotic drive element
• transmission ratio distorter

Related Terms

• Meiotic drive
• selfish genetic element
• toxin–antidote system
• suppressor of segregation distorter
• Rsp locus
• ultraconserved element
• Muller's ratchet

Usage Notes

Distinguish from general meiotic drive, which encompasses any non-Mendelian segregation bias; SD specifically refers to characterized systems with identified molecular mechanisms, particularly the Drosophila paradigm. The term is often used adjectivally ("SD chromosome," "SD system"). In arthropods, SD-like elements have been implicated in hybrid sterility, species boundary maintenance, and the evolution of genome structure. Researchers studying natural populations should note that SD activity can be suppressed by unlinked modifiers, so phenotypic ratios may not reveal underlying distortion frequencies.