Origins
The historic record associates the earliest documented generations of this matrilineage with the Darcy family.

From the finding that female descendants of the family 12 Brimmer Mare (MRCA - Spilletta 1749) and those of family 20 (MRCA - Mare 1765c by Locust) have the same mitochondrial DNA (mtDNA) haplotype, I2a2, it was concluded in a report published 2012(2) that Eclipse's maternal line descended from an "unknown Family 20 mare in Lord Montague's stable". This sweeping conclusion must be criticized on both scientific and historic grounds.

On the basis of equine mtDNA mutation rate estimates (1,3) it is reasonable to conclude that these two lineages share a common matrilineal ancestor that lived within the last 1000 years. However, the claim that both descend from the same 17th century mare cannot be justified from mtDNA findings alone i.e., without presentation of strong supporting evidence from the historic record. What little of that was presented in the report(2) is contradicted by multiple sources that apparently were not consulted.

As historical evidence, the authors offer only a pedigree for Eclipse that was published 76 years after he was foaled (Whyte, History of the British Turf 1840). It terminates in tail female line at his fourth dam, represented there as "old mare of Lord Montague's, pedigree unknown". Whyte offered no authority for this pedigree; it is therefore impossible to know whether he was privy to credible information or merely laboring under the same confusion between Eclipse's third dam by Darcy's Old Montagu and a matrilineally unrelated mare associated with Lord Montague of Cowdray that eventually resulted in conflation (GSB1, ed. 5) of two different matrilines into a single "family".

Eclipse's dam Spilletta can be traced to the Stud of the first Duke of Cumberland; her pedigree was given at the time she raced (Pond 1754) as "got by Regulus; her Dam was the Dam of Sir Robert Eaden's Mare, that won Hambleton in 1751 [Miss Western]; she was got by a Son of Snake, which was full Brother to the Williams's Squirrel, which was a famous Horse in the Year 1725, her Grandam was got by a Colt called Mountague, that was bred by Lord Darcy." Miss Western was also owned by H R H the Duke, but won her King's Plate (for 5 years old mares at Hambleton) for Sir Robert Eden; a sister was offered for sale after Sir Robert's death in 1755 [Newcastle Courant. No. 4111. 12 Jul 1755]. Two different pedigrees were given for the dam of Miss Western and Spilletta at least as late as 1785 or 1786 when Pick published his Authentic Historical Racing Calendar and was obliged to include a correction of Miss Western's pedigree in his "Errata and Additions." Most significantly, no claim was made in these contemporary records that would support the origin of Eclipse's matrilineal line in the Stud of Lord Montagu. In fact, the pedigree of Eclipse accepted by GSB continues with crosses of stallions associated with the Darcy family (Hautboy and Brimmer).

References
1. Achilli A, Olivieri A, Soares P, Lancioni H, Kashani BH, Perego UA, Nergadze SG, Carossa V, Santagostino M, Capomaccio S, Felicetti M, Al-Achkar W, Penedo M C, Verini-Supplize A, Houshmand M, Woodward S R, Semino, O, Silvestrelli M, Giuliotto E, Pereira L, Bandelt H J, Torroni A: Mitochondrial genomes from modern horses reveal the major haplogroups that underwent domestication. PNAS USA. 2012 Feb 14;109(7):2449-54

2. Bower M A, Campana M G, Nisbet R E R, Weller R, Whitten M, Edwards C J, Stock F, Barrett E, O’Connell T C, Hill E W, Wilson A M, Howe C J, Barker G, Binns M. Truth in the bones: Resolving the identity of the founding elite thoroughbred racehorses. Archaeometry. 2012[A]. doi:10.1111/j.1475-2012.00666.x

3. Vilstrup JT, Seguin-Orlando A, Stiller M, Ginolhac A, Raghavan M, et al. (2013) Mitochondrial Phylogenomics of Modern and Ancient Equids. PLoS ONE 8(2): e55950. doi:10.1371/journal.pone.0055950