CHROMOSOMAL EVOLUTION AND SPECIATION
127
arrangement) have existed in the lineage to which this species belongs
since it diverged from the main stock of 23- and 24-chromosome lycaenid
species. Each of these rearrangements was most likely a translocation
whereby a "donor" chromosome provided the 2 required telomeres together with some adjacent material. Whether such donor chromosomes
were supernumeraries of the type found in many grasshopper species,
heterochromatic members of the regular set, or even small euchromatic
elements, cannot be determined from the available evidence. But it is
clear that "fragmentation" on this scale must have involved a considerable increase in the total amount of genetic material in the karyotype,
and it seems likely that most of this duplicated material was heterochromatic.
An alternative, but much less probable interpretation of the Lysandra
case and other similar instances of greatly increased chromosome numbers in the Lepidoptera would be to suppose that each increase resulted from a single chromosomal break, the two fragments then forming isochromosomes as a result of sister-strand reunion. In most groups
of organisms isochromosomes (i.e., elements in which the sequence of
parts is ahecha)
cannot persist because they are usually dicentric or
acentric, and even if monocentrie (following a break through the centromere or immediately adjacent to it), the fact that their two limbs sometimes pair with one another at meiosis prevents them from being inherited in a regular manner. In a group with polycentric chromosomes
isochromosomes may stand a better chance of becoming established as
regular members of the karyotype, but one would expect that inter-arm
pairing would still constitute a difficulty. Another obstacle to the establishment of isochromosomes is, of course, that they upset the genie
balance of the individual, as pointed out by Svärdson (1945, p. 105),
who, however, seems not to regard this obstacle as a serious one. Whatever the precise mechanism of the "fragmentations" that have become
established in Lysandra, the meiotic mechanism must surely be such as
to permit the formation of trivalents with a regular "triangular" orientation on the first meiotic spindle, in both sexes (in some Lepidoptera
the chiasma frequency would probably be insufficient to permit trivalent
formation in the female, although it would be sufficient in the male).
In Lysandra there is no evidence that any chromosomal fusions have
occurred; the cytological evolution seems to have been unidirectional.
But in another genus of butterflies, Erebia, there are species with 8, 10,
11, 12, 14, 17, 19, 21, 22, 24, 28, 29, and 40 chromosome pairs (Lorkovic,
1941, 1949; Federley, 1938; de Lesse, 1953a). In this instance the species
E. ottomana, with η = 40, must have acquired at least 10 "fragmenta-
127
arrangement) have existed in the lineage to which this species belongs
since it diverged from the main stock of 23- and 24-chromosome lycaenid
species. Each of these rearrangements was most likely a translocation
whereby a "donor" chromosome provided the 2 required telomeres together with some adjacent material. Whether such donor chromosomes
were supernumeraries of the type found in many grasshopper species,
heterochromatic members of the regular set, or even small euchromatic
elements, cannot be determined from the available evidence. But it is
clear that "fragmentation" on this scale must have involved a considerable increase in the total amount of genetic material in the karyotype,
and it seems likely that most of this duplicated material was heterochromatic.
An alternative, but much less probable interpretation of the Lysandra
case and other similar instances of greatly increased chromosome numbers in the Lepidoptera would be to suppose that each increase resulted from a single chromosomal break, the two fragments then forming isochromosomes as a result of sister-strand reunion. In most groups
of organisms isochromosomes (i.e., elements in which the sequence of
parts is ahecha)
cannot persist because they are usually dicentric or
acentric, and even if monocentrie (following a break through the centromere or immediately adjacent to it), the fact that their two limbs sometimes pair with one another at meiosis prevents them from being inherited in a regular manner. In a group with polycentric chromosomes
isochromosomes may stand a better chance of becoming established as
regular members of the karyotype, but one would expect that inter-arm
pairing would still constitute a difficulty. Another obstacle to the establishment of isochromosomes is, of course, that they upset the genie
balance of the individual, as pointed out by Svärdson (1945, p. 105),
who, however, seems not to regard this obstacle as a serious one. Whatever the precise mechanism of the "fragmentations" that have become
established in Lysandra, the meiotic mechanism must surely be such as
to permit the formation of trivalents with a regular "triangular" orientation on the first meiotic spindle, in both sexes (in some Lepidoptera
the chiasma frequency would probably be insufficient to permit trivalent
formation in the female, although it would be sufficient in the male).
In Lysandra there is no evidence that any chromosomal fusions have
occurred; the cytological evolution seems to have been unidirectional.
But in another genus of butterflies, Erebia, there are species with 8, 10,
11, 12, 14, 17, 19, 21, 22, 24, 28, 29, and 40 chromosome pairs (Lorkovic,
1941, 1949; Federley, 1938; de Lesse, 1953a). In this instance the species
E. ottomana, with η = 40, must have acquired at least 10 "fragmenta-