(16) having been found in _Odontota dorsalis_, and the largest (40) in
_Silpha americana_. The difference in size is also very marked, as may
be seen by comparing the spermatogonial plates in figures 3 and 58 with
those shown in figures 94 and 141.
No other species of the Tenebrionidae has yet been secured, and all of
the other beetles examined differ in a marked degree from _Tenebrio
molitor_ in the growth stages of the spermatocytes. While in _Tenebrio_
the chromatin stains very dark throughout the growth stage, and the
unequal pair can not be distinguished until the prophase of division
('05, plate VI, figs. 171-180), in most of the others there are very
distinct synizesis and synapsis stages, following the last
spermatogonial mitosis, then a spireme stage in which the condensed
unequal pair of heterochromosomes or the odd chromosome is conspicuous
in contrast with the pale spireme, whether the preparation is stained
with iron-haematoxylin, gentian, or thionin. In _Tenebrio molitor_, the
unequal pair behaved in every respect like the other bivalent
chromosomes. In the other forms, though it behaves during the two
maturation divisions like the symmetrical bivalents, it remains
condensed during the growth period like the "accessory" of the
Orthoptera, the odd chromosome, "_m_-chromosomes," and "idiochromosomes"
of the Hemiptera. In several cases the heterochromosomes of the
Coleoptera are associated with a plasmosome (figs. 22, 23, 63, 132, 158,
217), as is often true in other orders. This peculiar pair of unequal
heterochromosomes varies considerably in size during the growth stage in
some of the species studied, but changes very little in form, differing
in this respect from the "accessory" in some of the Orthoptera (McClung,
'02) and from the large idiochromosome in some of the Hemiptera (Wilson,
'05).
The odd chromosome, so far as it has been studied, behaves precisely
like the larger member of the unequal pair without its smaller mate
(figs. 219, 220, 226, 233). In the growth stage it remains condensed and
either spherical or sometimes flattened against the nuclear membrane
(figs. 217, 225, 231). In the first maturation mitosis it is attached to
one pole of the spindle, does not divide, but goes to one of the two
second spermatocytes (figs. 233, 235). In the second spermatocyte it
divides with the other chromosomes, giving two equal classes of
spermatids differing by the presence or absence of this odd chromosome.
All of the evidence at hand leads to the conclusion that in the
Coleoptera, the univalent elements of all the pairs, equal and unequal,
separate in the first spermatocyte mitosis and divide quantitatively in
the second. In this respect the behavior of the chromosomes in this
order appears to be much more uniform than in the Orthoptera and
Hemiptera.
COMPARISON OF THE COLEOPTERA WITH THE HEMIPTERA AND LEPIDOPTERA.
As has been seen above, the conditions in the Coleoptera, so far as the
heterochromosomes are concerned, correspond very closely in final
results with those in the Hemiptera heteroptera and the Orthoptera. In
minor details these chromosomes are less peculiar in the Coleoptera than
in either of the other orders. Even condensation during the growth stage
is not universal, and synapsis of the heterochromosomes apparently
occurs simultaneously with that of the ordinary chromosomes, instead of
being delayed, as in many of the Hemiptera heteroptera.
_Aphrophora_ (Hemiptera homoptera) agrees with the _Anasa_ group of the
Hemiptera heteroptera in having a pair of condensed _m_-chromosomes, in
the growth stage, but this pair is already united in synapsis when first
seen. It differs from _Anasa_, but agrees with _Banasa_ and _Archimerus_
in exhibiting a typical odd chromosome which goes to one pole without
division in the first spermatocyte, and divides with the other
chromosomes in the second spermatocyte. The odd chromosome in this
species of Hemiptera, therefore, behaves like that in the Coleoptera and
Orthoptera. The most interesting points in the results of this study of
the germ cells of _Aphrophora_ is the discovery of two pairs of
condensed chromosomes in certain phases of the growth stages of the
oocytes. This has not been shown to be the case in any other species of
Hemiptera, so far as I can ascertain. It is now evident that in the
Heteroptera homoptera there are at least two distinct classes as to
behavior of chromosomes. In one class we have the Aphids (Stevens, '05
and '06) and Phylloxera (Morgan, '06) in which no heterochromosomes have
been found, while in the other class are such forms as Aphrophora with
both a pair of _m_-chromosomes and a typical odd heterochromosome.
The two species of Lepidoptera examined indicate that here we may have
conditions comparable to those in _Nezara_--an equal pair of
heterochromosomes whose only apparent peculiarity is their condensed
form during the growth stage. Doubtless the results of other
investigators will soon throw more light on the heterochromosomes of
this order.
GENERAL DISCUSSION.
It will be seen from the foregoing that the results obtained in the
study of the germ cells of _Tenebrio molitor_ have been confirmed in
full for several species of Coleoptera, and in part for 19[B] different
species belonging to 8[B] families. It has also been shown that a
different type of Coleopteran spermatogenesis exists in at least 3
families, where an odd chromosome like that in the Orthoptera occurs in
place of the unequal pair. In all of these insects the spermatozoa are
distinctly dimorphic, forming two equal classes, one of which either
contains one smaller chromosome or lacks one chromosome.
The most difficult part of the work has been the determination of the
somatic number of chromosomes in the male and female. In some cases
suitable material has been lacking; in others, though material was
abundant, no metaphases could be found in which the chromosomes were
sufficiently separated to be counted with certainty. In three species
(in addition to _Tenebrio molitor_) where the unequal pair is present,
the female somatic cells have been shown to contain the same number of
chromosomes as the spermatogonia, but an equal pair in place of the
unequal pair of the male. In two new cases the male somatic number and
size have been shown to be the same as in the spermatogonia. In one of
the Elateridae, where the spermatogonial number is 19, the female somatic
number is 20, and in _Aphrophora_ the numbers in male and female cells
are respectively 23 and 24. No exception has been found to the rule
established by previous work on the Coleoptera (Stevens, '05) and on the
Hemiptera (Wilson, '05 and '06), that (1) in cases where an unequal pair
is present in the male germ cells, it is also present in the male
somatic cells, but is replaced in the female by an equal pair, each
component being equal in volume to the larger member of the unequal
pair, and (2) in cases where an odd chromosome occurs in the male, a
pair of equal size are found in the female. It is therefore evident that
an egg fertilized by a spermatozoon (1) containing the small member of
an unequal pair or (2) lacking one chromosome, must develop into a male,
while an egg fertilized by a spermatozoon containing the larger element
of an unequal pair of heterochromosomes or the odd chromosome must
produce a female.
Whether these heterochromosomes are to be regarded as sex chromosomes in
the sense that they both represent sex characters and determine sex, one
can not decide without further evidence.
Comparison of the two types in Coleoptera, especially where, as in the
Carabidae, both occur in one family, has suggested to me that here it is
possible that the small chromosome represents not a degenerate female
sex chromosome, as suggested by Wilson, but some character or characters
which are correlated with the sex character in some species and not in
others. Assuming this to be the case, a pair of small chromosomes might
be subtracted from the unequal pair, leaving an odd chromosome. The two
types would then be reduced to one. It may be possible to determine the
validity of this suggestion for particular cases by observation or
experiment.
Since the first of this series of papers was published, there have
appeared three important papers by Prof. E. B. Wilson, bearing on the
problem of sex determination in insects. These papers are based on a
study of many species of the Hemiptera heteroptera. These insects fall
into two classes--one in which a pair of "idiochromosomes," usually of
different size, remain separate and divide quantitatively in the first
spermatocyte, conjugate and then separate in the second maturation
mitosis; and another class in which an odd chromosome--the
"heterotropic" chromosome--divides in one of the maturation mitoses, but
not in the other. Wilson regards the odd chromosome as the equivalent of
the larger of the "idiochromosomes," its smaller mate having
disappeared. In the somatic cells of the former class he finds in the
male the unequal pair, in the female an equal pair, the smaller
chromosome being replaced by an equivalent of the larger
"idiochromosome." In the latter class the male somatic cells contain the
odd number, the female somatic cells and oogonia an even number, the
homologue of the odd chromosome of the male being present and giving to
the female one more chromosome than are found in the male.
In his latest paper Wilson ('06) makes a variety of suggestions as to
sex determination. He shows that if the "idiochromosomes" and the
heterotropic chromosome be regarded as sex chromosomes in the double
sense that they both bear sex characters and determine sex, the
following scheme accounts for the observed facts in all cases where an
unequal pair or an odd heterochromosome have been found:
Sperm. Egg.
{Large [Male] "idiochromosome"}
I. {or } + Large [Female] sex chromosome = a [Female]
{Odd chromosome. }
II. {Small [Female] "idiochromosome"}
{or } + Large [Male] sex chromosome = a [Male]
{No sex chromosome }
Here we know that such a combination of gametes must occur to give the
observed results, but we are not certain that we have a right to
attribute the sex characters to these particular chromosomes or in fact
to any chromosomes. It seems, however, a reasonable assumption in
accordance with the observed conditions. The scheme also assumes either
selective fertilization or, what amounts to the same thing, infertility
of gametic unions where like sex chromosomes are present. It also
assumes that the large female sex chromosome is dominant in the presence
of the male sex chromosome, and that the male sex chromosome is dominant
in the presence of the small female sex chromosome. Or, it might rather
be said that these are not really assumptions, but inferences as to what
must be true if the heterochromosomes are sex chromosomes. This theory
of sex determination brings the facts observed in regard to the
heterochromosomes under Castle's modification of Mendel's Law of
Heredity ('99).
The question of dominance is a difficult one, especially in
parthenogenetic eggs and eggs which are distinctly male or female before
fertilization. It may be possible that the sex character of the egg
after maturation is always dominant in the fertilized egg, as appears to
be the case in these insects (see scheme). Conditions external to the
chromosomes may determine in certain cases, such as Dinophilus, which
sex character shall dominate in the growing oocyte, and maturation occur
accordingly. It is evident that this reasoning would lead to the
conclusion that sex is or may be determined in the egg before
fertilization, and that selective fertilization, or infertility of
gametic unions containing like sex characters, has to do, not with
actual sex determination, but with suitable distribution of the sex
characters to future generations. If both sex characters are present in
parthenogenetic eggs, as appears to be the case in aphids and
phylloxera, dominance of one or the other must be determined by
conditions external to the chromosomes, for we have both sexes at
different points in the same line of descent without either reduction or
fertilization.
Wilson suggests as alternatives to the chromosome sex determinant theory
according to Mendel's Law, (1) that the heterochromosomes may merely
transmit sex characters, sex being determined by protoplasmic conditions
external to the chromosomes; (2) That the heterochromosomes may be
sex-determining factors only by virtue of difference in activity or
amount of chromatin, the female sex chromosome in the male being less
active. The first of these alternatives is an attempt to cover such
cases as _Dinophilus_, _Hydatina_, and _Phylloxera_ with large female
and small male eggs. Here Morgan's ('06) suggestion as to degenerate
males seems much to the point. The male sex character, having become
dominant in certain eggs at an early stage, may, from that time on,
determine the kind of development. As to the second alternative, I see
no reason for supposing that the small heterochromosome of a pair is in
any different condition, as to activity, from the large one. The
condensed condition may not mean inactivity, but some special form of
activity. And, moreover, it has been shown that in certain stages of the
development of the oocyte of one form, _Aphrophora quadrangularis_,
there are pairs of condensed chromosomes corresponding to those of the
spermatocyte, so that there would hardly seem to be any basis for
Wilson's attempt to associate the difference in development of male and
female germ cells with activity or inactivity of chromosomes, as
indicated by condensed or diffuse condition of the chromatin.
On the whole, the first theory, which brings the sex determination
question under Mendel's Law in a modified form, seems most in accordance
with the facts, and makes one hopeful that in the near future it may be
possible to formulate a general theory of sex determination.
This work has been done in connection with a study of the problem of sex
determination, but, whatever may be the final decision on that question,
it brings together a mass of evidence in favor of the belief in both
morphological and physiological individuality of the chromosomes, as
advocated by Boveri, Sutton, and Montgomery. It also gives the strongest
kind of evidence that maternal and paternal homologues unite in synapsis
and separate in maturation, leaving the ripe germ cells pure with regard
to each pair of characters.
BRYN MAWR COLLEGE, _June 7, 1906_.
FOOTNOTES:
[B] AUG. 20, 1906.--36 species belonging to 12 families. See note, p.
49.
BIBLIOGRAPHY.
BOVERI, TH.
'02. Ueber mehrpolige Mitosen als Mittel zur Analyse des
Zellkerns. Verh. d. phys.-med. Ges. Wuerzburg, N. F., vol.
35.
CASTLE, W. E.
'03. The heredity of sex. Bull. Mus. Comp. Zool. Harvard
College, vol. 40, no. 4.
MCCLUNG, C. E.
'99. A peculiar nuclear element in the male reproductive
cells of insects. Zool. Bull., vol. 2.
'00. The spermatocyte divisions of the Acridiidae. Kans.
Univ. Quart., vol. 9, no. 1.
'01. Notes on the accessory chromosome. Anat. Anz., vol. 20,
nos. 8 and 9.
'02. The accessory chromosome--sex-determinant? Biol. Bull.,
vol. 3, nos. 1 and 2.
'02_a_. The spermatocyte divisions of the Locustidae. Kans.
Univ. Quart., vol. 1, no. 8.
'05. The chromosome complex of orthopteran spermatocytes.
Biol. Bull., vol. 9, no. 5.
MONTGOMERY, THOS. H., JR.
'01. A study of the chromosomes of the germ-cells of
Metazoa. Trans. Amer. Phil. Soc., vol. 20.
'03. The heterotypic maturation mitosis in Amphibia and its
general significance. Biol. Bull., vol. 4, no. 5.
'01_a_. Further studies on the chromosomes of the Hemiptera
heteroptera. Proc. Acad. Nat. Sci. Phila., 1901.
'04. Some observations and considerations upon the
maturation phenomena of the germ-cells. Biol. Bull., vol. 6,
no. 3.
'05. The spermatogenesis of _Syrbula_ and _Lycosae_ and
general considerations upon chromosome reduction and
heterochromosomes. Proc. Acad. Nat. Sci. Phila., 1905.
MORGAN, T. H.
'06. The male and female eggs of Phylloxerans of the
Hickories. Biol. Bull., vol. 10, no. 5.
NOWLIN, W. N.
'06. A study of the spermatogenesis of _Coptocycla
aurichalcea_ and _Coptocycla guttata_. Journ. of Exp. Zool.,
vol. 3, no. 3.
PAULMIER, F. C.
'99. The spermatogenesis of _Anasa tristis_. Journ. of
Morph., vol. 15.
DE SINETY.
'01. Recherches sur la biologie et l'anatomie des phasms. La
Cellule, vol. 19.
STEVENS, N. M.
'05. A study of the germ cells of _Aphis rosae_ and _Aphis
oenotherae_. Journ. of Exp. Zool., vol. 2, no. 3.
'05_a_. Studies in spermatogenesis, with especial reference
to the "accessory chromosome." Carnegie Inst. of Wash., pub.
no. 36.
'06. Studies on the germ cell of Aphids. Ibid., pub. no. 51.
SUTTON, W. S.
'02. On the morphology of the chromosome group in
_Brachystola magna_. Biol. Bull., vol. 4, no. 1.
'03. The chromosomes in heredity. Biol. Bull., vol. 4, no.
5.
WILSON, E. B.
'05. Studies on chromosomes. I. The behavior of the
idiochromosomes in Hemiptera. Journ. Exp. Zool., vol. 2, no.
3.
'05_a_. The chromosomes in relation to the determination of
sex in insects. Science, vol. 22, no. 564.
'05_b_. Studies on chromosomes. II. The paired
microchromosomes, idiochromosomes, and heterotropic
chromosomes in Hemiptera. Journ. Exp. Zool., vol. 2, no. 4.
'06. Studies on chromosomes. III. The sexual differences of
the chromosome-groups in Hemiptera, with some considerations
of the determination and inheritance of sex. Ibid., vol. 3,
no. 1.
DESCRIPTION OF PLATES
[The figures were all drawn with Zeiss oil-immersion 2 mm., oc. 12, and
have been reduced one-third, giving a magnification of 1,000 diameters.]
PLATE VIII.
_Trirhabda virgata_ (_Family Chrysomelidae_).
FIG. 1. Equatorial plate from somatic tissues of a male pupa, 27 large
chromosomes, 1 small one.
2. Equatorial plate from an egg follicle, 28 large chromosomes.
3. Equatorial plate of spermatogonium, 27 large chromosomes, 1 small
one.
4. First spermatocyte, synizesis stage.
5. First spermatocyte, early spireme stage, showing unequal pair of
chromosomes.
6-7. First spermatocyte, later growth stages.
8. First spermatocyte, prophase.
9-12. First spermatocyte, metaphase.
13. First spermatocyte, equatorial plate.
14-15. First spermatocyte, anaphase, showing separation of the elements
of the unequal pair (_l_ and _s_).
16. First spermatocyte, daughter plates.
17. Second spermatocytes, equatorial plates.
18. Second spermatocytes, equatorial plates showing V-shaped
chromosomes.
19. Second spermatocyte, early anaphase, the small chromosome in
metakinesis.
_Trirhabda canadense._
20. Equatorial plate from egg follicle, 30 large chromosomes.
21. Equatorial plate of spermatogonium, 29 large chromosomes, 1 small
one.
22. First spermatocyte, growth stage showing the heterochromosome group.
23. Heterochromosome group. _p_ = plasmosome, _l_ = large
heterochromosome, _s_ = small heterochromosome.
24-27. First spermatocyte, metaphase.
28. First spermatocyte, equatorial plate.
29. First spermatocyte, equatorial plate, small member of the unequal
pair only present.
30. First spermatocyte, daughter plates.
31. Second spermatocytes, equatorial plates.
32-33. Second spermatocytes, prophase.
[Illustration: STEVENS PLATE VIII
N. M. S. del.
COLEOPTERA]
PLATE IX.
_Chelymortha argus_ (_Family Chrysomelidae_).
FIGS. 34-35. Equatorial plates from egg follicles, 11 equal pairs, no
small chromosome.
36. Equatorial plate of spermatogonium, 21 large chromosomes, 1 small
one.
37. First spermatocyte, synizesis stage.
38-40. First spermatocyte, synapsis stage.
41-43. First spermatocyte, bouquet stage after synapsis.
44-45. First spermatocyte, spireme stage showing the unequal pair of
heterochromosomes.
46. First spermatocyte, prophase.
47-49. First spermatocyte, metaphase.
50-51. First spermatocyte, equatorial plates, _x_ the heterochromosome
pair.
52. First spermatocyte, showing metakinesis of the unequal pair.
53. First spermatocyte, anaphase.
54-55. Second spermatocyte, equatorial plates.
56. Second spermatocyte, anaphase.
_Odontota dorsalis_ (_Family Chrysomelidae_).
57. Equatorial plate of male somatic cell from walls of the testis, 15
large chromosomes, 1 small one.
58-59. Equatorial plates of spermatogonia, 15 large chromosomes, 1 small
one.
60. Resting nucleus of spermatogonium, showing plasmosome (_p_).
61. First spermatocyte, synizesis stage.
62. First spermatocyte, synapsis stage.
63-64. First spermatocyte, spireme stage, showing the larger and smaller
heterochromosome associated with a plasmosome.
65-68. First spermatocyte, prophases.
[Illustration: STEVENS PLATE IX.
N. M. S. del.
COLEOPTERA]
PLATE X.
_Odontota dorsalis._
FIGS. 69-70. First spermatocyte, metaphase.
71. First spermatocyte, equatorial plate.
72. First spermatocyte, metaphase, showing metakinesis of the
heterochromosomes.
73-74. First spermatocyte, anaphase.
75-76. Second spermatocyte, equatorial plates.
77. Second spermatocyte, showing metakinesis of the small chromosome
(_s_).
78. Second spermatocyte, prophase, showing chromosomes longitudinally
split.
79-80. Young spermatids, _n_ the chromatin nucleolus.
81-87. A series of stages in the development of the sperm head, showing
the various phases in the history of the chromatin nucleolus (_n_).
88. Cross-sections of nearly mature sperm heads.
89-90. Equatorial plates of spermatogonia of abnormal individual, 15
large chromosomes, 2 small ones.
91. First spermatocyte from same testis, spireme stage, showing 2 small
chromosomes associated with 1 large one and a plasmosome.
92. First spermatocyte from the same testis, metaphase showing a similar
heterochromosome group.
93. Second spermatocyte from same testis, equatorial plate, showing 2
small chromosomes.
_Epilachna borealis_ (_Family Coccinellidae_).
94. Equatorial plate of spermatogonium, 17 large chromosomes and 1 small
one.
95. First spermatocyte, spireme stage, showing the unequal pair.
96-97. First spermatocyte, late prophases.
98. First spermatocyte, metaphase, showing chromosomes of different
forms.
99-100. First spermatocyte, equatorial plate.
101. Unequal heterochromosome pair from a metaphase.
102. First spermatocyte, anaphase; ordinary chromosomes stippled to show
more clearly the metakinesis of the unequal pair.
103. Second spermatocyte, equatorial plates.
104. Second spermatocyte, prophase.
105-106. Abnormal giant spermatids, probably in process of degeneration.
_Euphoria inda_ (_Family Scarabaeidae_).
107. Equatorial plate of spermatogonium, 20 chromosomes. The 2 smallest
are the unequal pair of heterochromosomes (_l_ and _s_).
108. Resting spermatogonium, showing plasmosome (_p_).
109. First spermatocyte, spireme stage.
[Illustration: STEVENS PLATE X.
N. M. S. del.
COLEOPTERA]
PLATE XI.
_Euphoria inda._
FIGS. 110-111. First spermatocyte, prophases.
112-113. First spermatocyte, late prophase.
114-116. First spermatocyte, metaphase.
117. First spermatocyte, equatorial plate, _x_ the unequal pair.
118-120. First spermatocyte, anaphase.
121. First spermatocyte, daughter plates.
122. Second spermatocyte, prophase.
123. Second spermatocyte, equatorial plates.
124-125. Second spermatocyte, daughter plates of the two classes.
126-127. Second spermatocyte, anaphase.
128-130. Spermatids, _n_ the chromatin nucleolus.
_Blepharida rhois_ (_Family Chrysomelidae_).
131-132. First spermatocyte, spireme stages, showing the
heterochromosome group.
133-135. First spermatocyte, beginning of metakinesis.
136. First spermatocyte, equatorial plate, _x_ the unequal pair.
137. First spermatocyte, late anaphase, showing the heterochromosomes
_l_ and _s_.
138. Second spermatocyte, equatorial plates.