Partner telomeres during anaphase in crane-fly spermatocytes are connected by an elastic tether that exerts a backward force and resists poleward motion
James R. LaFountain, Jr1,*,
Richard W. Cole2 and
Conly L. Rieder2,3
1 Department of Biological Sciences, 657 Cooke Hall, University at Buffalo,
Buffalo, NY 14260-1300, USA
2 Laboratory for Cell Regulation, Division of Molecular Medicine, Wadsworth
Center for Laboratories and Research, New York State Department of Health, PO
Box 509, Albany, NY 12201-0509, USA
3 Department of Biomedical Sciences, State University of New York, Albany, NY
12222, USA
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Fig. 1. Transient stretching of trailing chromosome arms is common during anaphase
in crane-fly spermatocytes. (A-D) Selected frames from a time-lapse DIC
recording of a primary spermatocyte in which arms of segregating partner
half-bivalents stretched backwards during mid-anaphase. (A) The dichiasmic
bivalent at metaphase. (B) The two half-bivalents began to segregate. (C) As
anaphase progressed, trailing arms (arrowhead) of partners stretched
backwards. (D) Partner arms retract as the half-bivalents continue moving
polewards. Times are given in minutes and seconds. Bar, 5 µm (D). (E-H)
Spermatocytes that were fixed in situ with glutaraldehyde. In each, arms of
partner chromosomes are stretched backwards as if their telomeres were
connected. The sex chromosomes normally lag at the spindle equator during
anaphase in these cells and they are especially prominent in G and slightly in
focus in E, F and H. Bar, 5 µm (H).
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Fig. 2. Backward forces on trailing arms cause a detached acentric arm fragment to
move towards its partner. (A-D) Selected frames from a time-lapse series of an
operation that generated an acentric fragment from a trailing arm during
anaphase. (A) Before the operation. (B) Following the operation, the acentric
fragment (arrowhead) moved backwards towards its partner with an initial
velocity of  8 µm/minute. (C) The fragment crossed the equator and as
its velocity decreased, it made contact with its partner (D). (E) As anaphase
progressed, the fragment moved along with its partner to the opposite pole.
Times are given in minutes and seconds. Bar, 5 µm (E). (F) A kinetic plot
of the distance between the telomere of an acentric arm fragment (not
depicted) and its partner telomere following laser microsurgery (arrow) as a
function of time. The backward motion of the tethered fragment exhibited
decreased velocity as the fragment approached its partner in the opposite half
spindle prior to making contact, when the distance between telomeres became
zero.
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Fig. 3. The movement of an acentric fragment generated by cutting a trailing arm
from a segregating half-bivalent conformed to one of three types. (A-C) The
single trailing arm that was to be severed is represented by an open rectangle
and the acentric fragment generated by cutting is represented by an open
square. The kinetochore-containing fragment is represented by an open square
connected to a circle, the latter representing the centromere/kinetochore
domain of the half-bivalent. For simplicity, only the to-be-cut arm and its
partner are included. (A) Type 1 fragment: the fragment moved rapidly
backwards across the equator into the opposite half spindle (A2) to make
contact with its partner (A3) and then moved along with its partner to the
lower pole (A4). The tethered arm of the partner half-bivalent is shown with a
solid rectangle and solid circle. (B) Type 2 fragment: the fragment moved
backwards across the spindle into the opposite half spindle (B2), but it did
not make contact with its partner (B3) as in (A). Further movement of the
fragment to the opposite pole occurs after the partner (solid) reaches the
pole (B4). (C) Type 3 fragment: this fragment moved backwards a short distance
(or did not move backwards at all) without crossing the equator (C2) then
shifted from backward movement to movement to the correct pole (C3) and at
late anaphase it was located near its original pole (C4).
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Fig. 4. Backward movement of an acentric fragment was mediated by its telomere.
(A-D) Selected frames from a time-lapse recording of a double-cut operation.
(A) Before the first cut. (B) After the first cut, the fragment (arrow) moved
backwards. (C) After the second cut, the telomere-containing fragment
(arrowhead) continued moving backwards, but the interstitial fragment (arrow)
halted at the location of the second cut. Times are given in minutes and
seconds. Bar, 5 µm (D).
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Fig. 5. Backward movement of a fragment halted upon irradiation of the telomere of
its partner. (A-B) Selected frames from a recording of a telomere ablation
operation. (A) Before the operation. (B) The acentric fragment (arrowhead)
moved backwards rapidly and was probably a type 1 fragment
(Fig. 3A). (C) After the
telomere (arrow) of the fragment's partner was irradiated, the backward
movement of the fragment stopped (D). Times are given in minutes and seconds.
(E) As anaphase progressed, both partners continued polewards, but the
transport properties of the spindle (see text) began to act on the fragment
(arrowhead) and moved it away from the equator and into the half-spindle. Bar,
5 µm (E).
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Fig. 6. Kinetochore detachment operations generated both tethered and untethered
fragments. (A-E) Selected frames from a time-lapse recording of a kinetochore
detachment operation. (A) Before the operation. (B) The K-fragment (white
arrowhead) was detached and continued moving polewards; four acentric
fragments were generated and the telomeres of two of them are located with
black arrowheads. (C) The K-fragment (white arrowhead) approaches the spindle
pole; the tethered, type 2 fragment (arrow points to its telomere) exhibited
rapid backward motion; among the other three fragments were a weakly tethered
fragment (left-pointing arrowhead) and two untethered fragments (the telomere
of one is located with a right-pointing arrowhead). (D) The positions of all
four fragments generated by laser microsurgery are evident: the arrow locates
the telomere of the type 2 fragment, one left-pointing arrowhead locates the
telomere of the weakly tethered fragment near the equator and the other two
arrowheads locate the telomeres of the untethered fragments, which are in the
process of being transported to the lower pole. Times are given in minutes and
seconds. Bar, 5 µm (D).
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Fig. 7. Backward forces were sufficiently strong to achieve the backward movement
of an entire acentric half-bivalent. (A-E) Selected frames from a time-lapse
recording of an operation in which the kinetochores were removed from a
segregating half-bivalent. Here the acentric arms remained `stuck' together
following the operation and moved backward as a unit, in this case with an
initial velocity of 2 µm/minute. (A) Before the operation. (B) After
the operation, the removal of kinetochores is evident from the truncated
appearance of the half-bivalent; the operation also generated a ribbon of
denatured nucleoprotein, called a sniglet (arrowhead). (C) The sniglet
(arrowhead) was transported polewards, but the truncated half-bivalent moved
backwards towards its partner. (D) The acentric arms continued moving towards
the partner's pole, as the sniglet (arrowhead) was transported further
polewards. (E) At late anaphase, the truncated half-bivalent was located in
the opposite half-spindle and the sniglet (arrowhead) was located near the
spindle pole. Times are given in minutes and seconds. Bar, 5 µm (E).
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Fig. 8. Severing of trailing arms during meiosis II also resulted in backward
movement of acentric fragments. (A-D) Selected frames from a time-lapse
recording of a secondary spermatocyte undergoing anaphase II. (A) Sister
chromatids segregating to opposite poles before the operation. (B) A trailing
arm (arrowhead) of one of the sex chromosomes was cut. (C) The acentric
fragment (arrowhead) moved backwards to make contact with its sister. (D) The
fragment moved along with its sister as anaphase was completed. Times are
given in minutes and seconds. Bar, 5 µm (D).
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© The Company of Biologists Ltd 2002
